utimens: work around older Linux failure with symlinks

utimens: work around older Linux failure with symlinks

Some Linux kernel versions support utimensat(,0) but not
utimensat(,AT_SYMLINK_NOFOLLOW), even for non-symlinks.  Had the
fallback code for regular files been reached, it would have truncated
timestamps.  But since glibc lutimes just wraps utimensat, and we
didn't expect ENOSYS from lutimes, the fallback wasn't even reached.

* lib/utimens.c (lutimensat_works_really): New variable.
(fdutimens, lutimens): Use it to manage kernels that support
nanosecond times on files, but not on symlinks.
Reported by Ondřej Vašík.

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