chown,chgrp: reinstate numeric id output in -v messages

chown,chgrp: reinstate numeric id output in -v messages

since gnulib commit ff208d546a,
related to coreutils commit v9.0-143-gabde15969
we no longer maintain numeric IDs through chopt->{user,group}_name.
Therefore we need to adjust to ensure tests/chown/basic.sh passes.

* src/chown-core.c (uid_to_str, gid_to_str): New helper functions
to convert numeric id to string.
(change_file_owner): Use the above new functions to pass
numeric ids to describe_change().