Add support for Unix groups comparable to user id

Add support for Unix groups comparable to user id

	* src/Makefile.am (libsxecore_a_headers):Add new sysgrp.h header.

	* src/sysgrp.h: New header

	* src/editfns.c: Include sysgrp.h

	* src/editfns.c (Fuser_group_name): New defun similar to
	Fuser_login_name but for groups.

	* src/editfns.c (user_group_name): Actual implementation of
	user_group_name. Can be called by other C functions in SXEmacs.

	* src/editfns.c (Fuser_gid): Group counterpart to user-uid

	* src/editfns.c (Fuser_real_gid): Group counterpart to
	user-real-uid

	* src/editfns.c (syms_of_editfns): Register new defuns
	Fiser_group_name, Fuser_gid, Fuser_real_gid

	* src/lisp.h: Add prototype for internal user_group_name

	* src/lisp.h: Add declaration of Fuser_group_name

Signed-off-by: Nelson Ferreira <nelson.ferreira@ieee.org>