/* call-seq:
 *    Admin.get_group(name)
 *    Admin.get_group(gid)
 *
 * Returns a Group object for the given +name+ or +gid+. Raises an Admin::Error
 * if a group cannot be found for that name or GID.
 *
 *--
 * Developer's Note:
 *
 * I generally oppose method overloading like this, but for this method, and
 * for only two types, I can live with it for the added convenience it
 * provides.
 */
static VALUE admin_get_group(VALUE klass, VALUE v_value){