futimens, utimensat: work around Linux bug

futimens, utimensat: work around Linux bug

futimens is trivial - let fdutimens do the work.  utimensat
is tougher: we don't want to call into local_utimensat,
because that can cause unnecessary chdir.  So we have to
repeat the logic from utimens.c.

* m4/futimens.m4 (gl_FUNC_FUTIMENS): Detect ctime bug.
* m4/utimensat.m4 (gl_FUNC_UTIMENSAT): Likewise.
* lib/utimensat.c (rpl_utimensat): Work around it.
* lib/futimens.c (rpl_futimens): Adjust comment.