doc: cover st_ino issues once; add OpenVMS etc.

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.