Allow optional/required UF account creation through profile

Change the usage of civicrm_uf_group.is_cms_user - which is now a boolean. Instead, 0 = no acct create option; 1 = give option, but not required; 2 = account creation required (if user not logged in, of course). Then change UI to radio button (instead of checkbox). Change profile form rendering to handle the "required" case (no checkbox for user, username required etc.