chgrp: add --from parameter similar to chown

chgrp: add --from parameter similar to chown

* doc/coreutils.texi (chown invocation): Convert --from option
description to a macro and call from ...
(chgrp description): ... here.
* src/chown-core.h (emit_from_option_description): A new function
refactored from ...
* src/chown.c (usage): ... here, and called from ...
* src/chgrp.c (usage): ... here.
(main): Accept the --from option as chown(1) does.
* po/POTFILES.in: Add chown-core.h as now translated.
* tests/chown/basic.sh: Decouple the root user from id 0.
* tests/chgrp/from.sh: A new test largely based on chown/basic.sh.
* tests/local.mk: Reference the new test.
* NEWS: Mention the new feature.
Suggested by Ed Neville.