cp -pP (and e.g., -a): preserve time stamps on symlinks, too

cp -pP (and e.g., -a): preserve time stamps on symlinks, too

* src/copy.c (utimensat_if_possible): New function.
(copy_internal): Remove variable, "preserve_metadata".
Replace with "dest_is_symlink".  That covers all cases but one:
the one in which cp --link has created hard links to non-directories.
In that case, there is no need to update attributes of the links.
Use utimensat_if_possible, to preserve timestamps of symlinks.
* NEWS (New features): Mention this.
* tests/Makefile.am (TESTS): Add cp/preserve-slink-time.
* tests/cp/preserve-slink-time: New file.
* m4/jm-macros.m4 (coreutils_MACROS): Test for utimensat.
Reported in http://bugzilla.redhat.com/230866