grp: implement POSIX group APIs.

Closes #151.
* specs/posix_grp_spec.yaml (endgrent, getgrent, getgrgid)
(getgrnam, setgrent): Specify correct behaviours.
* ext/posix/grp.c (endgrent, getgrent, getgrgid, getgrnam)
(setgrent): Implement bindings to these POSIX group APIs.
* specs/posix_spec.yaml (getgroup): Specify expected behaviour.
* ext/posix/grp.c (getgroup): Remove.
* lib/posix.lua (getgroup): Reimplement over POSIX APIs to
continue satisfying specifications.
* NEWS: Update.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>