copy: fix SELinux context preservation for existing directories

copy: fix SELinux context preservation for existing directories

* src/copy.c (copy_internal): Use the global process context
to set the context of existing directories before they're populated.
This is more consistent with the new directory case, and fixes
a bug for existing directories where we erroneously set the
context to the last copied descendent, rather than to that of
the source directory itself.
* tests/cp/cp-a-selinux.sh: Add a test for this case.
* NEWS: Mention the fix.
* THANKS.in: Add reporter Michal Trunecka.