utimensat: new module

Provide utimensat where it is missing, and rpl_utimensat to work
around ENOSYS and EINVAL bugs in older Linux kernels.

* modules/utimensat: New file.
* lib/utimensat.c (utimensat): Likewise.
* m4/utimensat.m4 (gl_FUNC_UTIMENSAT): Likewise.
* lib/utimens.c (utimensat): Avoid recursion into rpl_utimensat,
so we can work around Linux bugs.
* m4/sys_stat_h.m4 (gl_SYS_STAT_H_DEFAULTS): Add witnesses.
* modules/sys_stat (Makefile.am): Substitute them.
* lib/sys_stat.in.h (utimensat): Declare it.
* MODULES.html.sh (systems lacking POSIX:2008): Mention module.
* doc/posix-functions/utimensat.texi (utimensat): Likewise.
* modules/utimensat-tests: New test.
* tests/test-utimensat.c: Likewise.

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