TECHNICAL FIELD The present invention relates to a modular tubular member, such as a fishing rod or windsurf board mast. The tubular member according to the invention is made in two or more segments joined by fitting one or more connecting pegs inside. BACKGROUND ART Various applications employ tubular modular members comprising two or more selectively connectable segments for easy transport and replacement of the various segments. The tubular members used for fishing rods and windsurf board masts are mostly made of composite material, such as carbon fibre, glass fibre, or similar, and the segments are joined by means of pegs partly fitted axially inside a first segment and partly inside a second segment to be joined to the first, so as to form an interference fit between the two segments. At the join, the connected segments have respective mating surfaces contacting one another and perpendicular to the longitudinal axis of the tubular member. The main drawback of known tubular members of this sort lies in the difficulty encountered in positioning the segments angularly -as required, and in maintaining the correct angular position even during use. For fishing rods to function properly, for example, it is essential that the slip rings, through which the line runs, be aligned properly, and this can only be done by connecting the segments of the rod in a given angular position. In some applications, to ensure correct positioning, the segments have reference marks which, when aligned, define the correct angular position in which to connect the segments. This solution, however, while ensuring correct angular positioning of the segments when assembling the tubular member, does not solve the problem of maintaining that position when actually using the tubular member, there being no means by which to effectively prevent relative rotation of the segments. This is a serious problem in the case of fishing rods, in particular, in which the considerable stress to which the rod is subjected, e.g. by pull on the line, may result in relative rotation of the segments. DISCLOSURE OF INVENTION It is therefore an object of the present invention to provide a modular tubular member, such as a fishing rod, windsurf board mast, or similar, comprising a number of segments that can be connected quickly and easily in a given angular position, which is maintained in use, even in the presence of severe stress. The present invention is defined and characterized substantially in the main Claim. Other innovative characteristics of the invention are defined in the secondary Claims. The modular tubular member according to the present invention comprises a number of segments joined by means of connecting pegs inserted axially inside. At at least one end, each segment has a mating surface which, when assembling the tubular member, rests against a joined mating surface formed at at least one end of another segment to be joined to it. According to one characteristic of the present invention, each of the mating surfaces is so formed as to intersect at least two different planes offset with respect to each other and perpendicular to the longitudinal axis of the tubular member. The axial distance between the two perpendicular planes is proportional to the diameter, or transverse dimension, of the tubular member at the join between the segments, and depends on the desired relative torsional resistance of the segments for connection. The axial distance is advantageously at least 0.3- 0.4 times the diameter, or minor transverse dimension, of the tubular member at the join between the segments. The conformation of the mating surfaces defines, between the segments of the tubular member, at least one predetermined angular connecting position, and substantially prevents one segment rotating with respect to the other when the segments are joined, even if one of them is subjected to a twisting' moment which would tend to rotate it with respect to the other. In a preferred embodiment of the invention, the conformation of the mating surfaces defines a foolproof angular connecting position of the segments. In one embodiment, each mating surface is a continuous surface sloping with respect to the longitudinal axis of the tubular member. In a further embodiment, the mating surface is a discontinuous surface having at least one portion sloping with respect to the longitudinal axis of the tubular member. In one variation, the mating surface is wedge-shaped and defined by two symmetrical portions sloping with respect to the longitudinal axis of the tubular member. In a further variation, the mating surface is defined by at least one sloping portion and by a portion perpendicular to the longitudinal axis of the tubular member. In a further variation, the mating surface has at least one portion parallel and two portions perpendicular to the longitudinal axis of the tubular member. In a further variation, the mating surface has at least one undulated portion. In a further variation, the mating surface is serrated. BRIEF DESCRIPTION OF THE DRAWINGS These and other characteristics of the present invention will be made clear in the following description of a number of preferred, non-limiting embodiments described with reference to the accompanying drawings, in which: Figure 1 shows a front view and enlarged detail of a fishing rod featuring a tubular member in accordance with the present invention; Figure 2 shows a partly sectioned, exploded view of two segments of the Figure 1 tubular member and a connecting peg; Figure 3 shows the Figure 2 segments joined; Figure 4 shows a partial isometric view of a segment of a tubular member in accordance with the invention; Figures 5, 6, 7, 8 show variations of Figures 1, 2, 3, 4 respectively; Figures 9, 10, 11 show further variations of the present invention. BEST MODE FOR CARRYING OUT THE INVENTION Number 10 in the accompanying drawings indicates as a whole a modular tubular member in accordance with the present invention and defining, in the example shown, a fishing rod 11. Tubular member 10 comprises four interchangeable, axially hollow segments 12, 13, 14, 15, which can be joined selectively in series into the work configuration of tubular member 10 (Figure 1), and can be disassembled for easy transport. Each two adjacent segments, e.g. hollow intermediate segments 13 and 14, are joined by means of a peg 16, substantially half of which is inserted inside one hollow segment 13, and the other half of which is inserted inside the other hollow segment 14 to lock the two segments to one another by inference (Figures 2-3 and 6- 7) . According to the invention, each segment 12, 13, 14, 15 has, at at least one of its two ends, a mating surface 18 which extends between two offset planes α and β perpendicular to the longitudinal axis "y" of tubular member 10. In actual use, a seal (not shown) of rubber or soft plastic may be provided on peg 16 and gripped between the two mating surfaces 18 to permit a minimum amount of rotation of segments 13, 14, by which to apply leverage to disconnect segments 13, 14. The axial distance "1" between planes α and β is proportional to the diameter "d" of tubular member 10, i.e. of relative segments 12, 13, 14, 15, at the join between the segments (Figures 1 and 5) . Axial distance "1", however, varies as a function of the desired relative torsional resistance of segments 12, 13, 14, 15. Advantageously: 1 = (0.3 to 0.4)d. In the Figure 1-4 embodiment, each segment 12, 13, 14, 15 has, at at least one of its two ends, a mating surface 18 sloping with respect to longitudinal axis "y" by an angle equal to the slope angle of a mating surface 18 of an adjacent segments 12, 13, 14, 15. More specifically, the bottom and. top segments 12 and 15 each have one mating surface 18, at the top and bottom end respectively, while intermediate segments 13 and 14 each have two mating surfaces 18, one at each end. To join two adjacent segments, e.g. intermediate segments 13 and 14, the relative mating surfaces 18 are positioned parallel to each other (Figure 2), and, once peg 16 is inserted, are brought to rest one against the other (Figure 3) to define the correct angular connecting position of segments 13 and 14, in which the slip rings 17 of fishing rod 11 are aligned. Segments 13 and 14 are thus connected integrally to each other, so that, during use, even if subjected to severe twisting forces, they are prevented from rotating with respect to each other, unless axial pull is first applied. In the Figure 5-8 embodiment, respective mating surfaces 18a or 18b of segments 12, 13, 14, 15 are wedge- shaped, each defining two specular portions 19a sloping with respect to longitudinal axis "y" of tubular member 10. More specif ically, in the case of two adj acent segments, one, e.g. intermediate segment 13, has a convex wedge-shaped mating surface 18a, while the other, e.g. intermediate segment 14, has a concave wedge-shaped mating surface 18b which mates with mating surface 18a when assembling tubular member 10. Figures 9, 10, 11 show further variations of mating surfaces 18 of segments 13 and 14 of tubular member 10 according to the invention. In Figure 9, mating surfaces 18 of segments 13 and 14 each comprise a sloping portion 19b extending between planes α and β; and a portion 19c perpendicular to longitudinal axis "y" of tubular member 10. In the Figure 10 embodiment, mating surfaces 18 of segments 13 and 14 each comprise two offset portions 19d perpendicular to axis "y"; and one portion 19e parallel to longitudinal axis "y" of tubular member 10. In the Figure 11 embodiment, mating surfaces 18 of segments 13 and 14 each have an undulated conformation extending between planes α and β. Clearly, changes may be made and/or parts added to tubular member 10 as described herein without, however, departing from the scope of the present invention. For example, it may be other than circular in cross section, and of constant cross section along the whole of its length. In which case, axial distance "1" between planes α and β is proportional to the transverse dimension of tubular member 10. Mating surfaces 18 of segments 12, 13, 14, 15 may be shaped differently from those in the embodiments shown. In one possible embodiment, mating surfaces 18 may be defined by respective irregular teeth defining a foolproof angular connecting position of the segments, or by respective regular teeth defining a number of angular connecting positions of the segments, while still preventing relative rotation of the segments.