utimens: add UTIME_NOW and UTIME_OMIT support

utimens: add UTIME_NOW and UTIME_OMIT support

These flags make it possible to implement futimens and utimensat;
they also make touch(1) more efficient, by avoiding stat or
gettime if native utimensat works.

* lib/utimens.c (validate_timespec, update_timespec): New helper
functions.
(gl_futimens, lutimens): Use them.
* modules/utimens (Depends-on): Add gettime, lstat, stat-time,
stdbool, sys_stat.
(Link): Mention resulting library dependency.
* modules/utimecmp (Link): Likewise.
* modules/utimens-tests (Depends-on): Drop stat-time, stdbool.
(Makefile.am): Pick up library dependency.
* lib/sys_stat.in.h (UTIME_NOW, UTIME_OMIT): Guarantee a
definition.
* tests/test-sys_stat.c: Test the definitions.
* doc/posix-headers/sys_stat.texi (sys/stat.h): Document this.
* NEWS: Document library dependency.

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