utimens: add lutimens interface

utimens: add lutimens interface

Wraps utimensat(,AT_SYMLINK_NOFOLLOW) or lutimes, when supported;
otherwise fail with ENOSYS.  Allows coreutils' copy.c to preserve
symlink timestamps on more systems.  Note that cygwin's lstat
changes atime of symlinks, but mtime can reliably be set.

* lib/utimens.c (lutimens): New function.
* m4/utimens.m4 (gl_UTIMENS): Check for lutimes.
* lib/utimens.h (lutimens): Declare new interface.
* tests/test-utimens.c (main): Enhance test.
* tests/test-lutimens.h (test_lutimens): New file.
* modules/utimens-tests (Files): Distribute it.
(Depends-on): Add symlink.
(configure.ac): Check for usleep.

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