added truncate(2), ftruncate(2) as file_truncate() and file->truncate()

Rev: src/fdlib.c:1.25
Rev: src/fdlib.h:1.24
Rev: src/modules/files/configure.in:1.66
Rev: src/modules/files/efuns.c:1.69
Rev: src/modules/files/file.c:1.156
Rev: src/modules/files/file_functions.h:1.13
Rev: tutorial/tutorial.wmml:1.107