cp: with --force; replace self referential symlinks

cp: with --force; replace self referential symlinks

* src/copy.c (copy_internal): Don't fail immediately upon
getting ELOOP when running stat() on the destination,
rather proceeding if -f specified, allowing the link
to be removed.  If the loop is not in the final component
of the destination path, we still fail but at the
subsequent unlink() stage.
* doc/coreutils.texi (cp invocation): Adjust wording to say
that --force doesn't work with dangling links, rather than
all links that can't be traversed.
* tests/cp/thru-dangling.sh: Add a test case.
* NEWS: Mention the change in behavior.
Discussed in https://bugs.gnu.org/31335