Move authentication return codes to gw_authenticator.h

The MYSQL_* authentication return codes are now in gw_authenticator.h so
that all authenticators can use them. Also dropped the MYSQL_ prefix from
the return codes and added AUTH_INCOMPLETE for a generic
authentication-in-progress return code.

server/core/users.c

@@ -224,5 +224,5 @@ int users_default_loadusers(SERV_LISTENER *port)
 {
     users_free(port->users);
     port->users = users_alloc();
-    return AUTH_LOADUSERS_OK;
+    return MXS_AUTH_LOADUSERS_OK;
 }