maint: merge chgrp and chown sources
chown is a close superset of chgrp functionality,
so merge sources to avoid unwanted divergence in future.
This removes about 300 lines in chgrp.c
* build-aux/gen-single-binary.sh: Generate new rules for chgrp.
* cfg.mk: Exclude new wrappers.
* po/POTFILES.in: Remove chgrp.c
* src/chgrp.c: Remove.
* src/chown-chgrp.c: New wrapper.
* src/chown-chown.c: Likewise.
* src/chown.c (main): Prepend ':' for chgrp(1).
* src/chown.h: Define both operating modes.
(usage): Adjust depending on utility being called.
* src/coreutils-chgrp.c: Likewise.
* src/local.mk: Reference new wrappers.