cp: make --attributes-only override --reflink completely

cp: make --attributes-only override --reflink completely

* doc/coreutils.texi (cp invocation): Change the description slightly
so as users might not immediately discount using this option.
Mention that --reflink is overridden by the other linking options and
--attributes-only, and give an example where this might be useful.
* src/copy.c (copy_internal): Bypass the reflink if
--attributes-only is specifed.
* tests/cp/reflink-perm: Ensure both --reflink modes are
overridden by --attributes-only.
* NEWS: Mention the change in behavior.
Reported by Jim Meyering.