Only allow enabled groups as primary group

Only allow enabled groups as primary group

So far any group in the user's token could be used as primary group.
Windows doesn't check if the primary group is enabled or not, it just
has no meaning.  From a POSIXy point of view it can lead to weird
results though.

	* uinfo.cc (check_token_membership): New static function.
	(internal_getlogin): Only allow enabled groups as primary group.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>