doc: cover st_ino issues once; add OpenVMS etc. * doc/posix-functions/stat.texi (stat): * doc/posix-functions/lstat.texi (lstat): * doc/posix-functions/fstatat.texi (fstatat): * doc/posix-functions/fstat.texi (fstat): Move general 'struct stat' stuff to sys_stat.texi, leaving behind a pointer. * doc/posix-headers/sys_stat.texi (sys/stat.h): Merge duplicate info about 'struct stat' problems into here. Mention issues with OpenVMS, GNU/Linux NFS, NetApp, ClearCase, and suggest partial workarounds.