posix: getgroup argument defaults to egid.

* specs/posix_spec.yaml (getgroup): Specify behaviour with no
arguments.
* lib/posix.lua.in (group): New LDocs for getgroup return table
fields.
(getgroup): Adjust LDocs accordingly.
Default to egid with no argument.
* NEWS: Update.

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