Change auditon(2) so that if somebody supplies an invalid command, it

returns EINVAL. Right now we return 0 or success for invalid commands,
which could be quite problematic in certain conditions.

MFC after:	1 week
Discussed with:	rwatson

sys/security/audit/audit_syscalls.c

@@ -406,6 +406,9 @@ auditon(struct thread *td, struct auditon_args *uap)
 		    (udata.au_trigger > AUDIT_TRIGGER_MAX))
 			return (EINVAL);
 		return (audit_send_trigger(udata.au_trigger));
+
+	default:
+		return (EINVAL);
 	}
 
 	/*