utimecmp: support symlink timestamps

utimecmp: support symlink timestamps

Update this module to reflect POSIX 2008:
With pathconf, we can avoid calling utimens.
With utimensat, the system resolution can be 1.
With lutimens, we can determine resolution on symlinks.

* lib/utimecmp.c (utimecmp): Use new interface.  Skip effort of
hashing when possible.  Use pathconf when available.
(SYSCALL_RESOLUTION): Recognize tighter resolution.
* modules/utimecmp (Depends-on): Add lstat.

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