Make chgrp and chown diagnostics consistent.

Make chgrp and chown diagnostics consistent.
* src/chown.c (main): Emit the diagnostic before the file name,
not after it, to be consistent with chgrp's diagnostic.
* src/chgrp.c (parse_group): Emit a ":" between the diagnostic
and the file name.
Reported by Egmont Koblinger.
* THANKS: Add Egmont Koblinger.