doc: mention timestamp portability issues

doc: mention timestamp portability issues

* doc/glibc-functions/lutimes.texi (lutimes): Refer to utimensat
instead.
* doc/posix-functions/utime.texi (utime): Likewise.
* doc/posix-functions/utimes.texi (utimes): Likewise.
* doc/glibc-functions/futimes.texi (futimes): Refer to futimens
instead.
* doc/posix-functions/futimens.texi (futimens): Mention utimens
module.
* doc/posix-functions/utimensat.texi (utimensat): Likewise.
Mention weakness with symlink timestamps.
* doc/glibc-functions/futimesat.texi (futimesat): New file; refer
to utimensat/futimens instead.
* doc/gnulib.texi (Glibc sys/time.h): Include new file.

Signed-off-by: Eric Blake <ebb9@byu.net>