Clean Up Form_alter

I've created a patch that cleans up the mailman_groups_form_alter function a little. It fixes an issue I was having with the unlink button not working. It also cleans up the form so it's a little more readable to the developer, and doesn't have to create a function to update/unlink existing groups from a mailing list as it finds the gid in $form_state['clicked_button']['#id'].