Make login_style private to bsdauth.c

Add a setter for policy.c to handle auth_type from the front-end.

plugins/sudoers/sudoers.h

@@ -324,7 +324,6 @@ int pam_prep_user(struct passwd *);
 
 /* gram.y */
 int sudoersparse(void);
-extern char *login_style;
 extern bool parse_error;
 extern bool sudoers_recovery;
 extern bool sudoers_strict;