Add authentication plugin name to authenticator API

The authenticators can now declare the authentication plugin name. Right
now this is only relevant for MySQL authentication but for example the
HTTP module could implement both Basic and Digest authentication.

server/modules/authenticator/max_admin_auth.c

@@ -63,7 +63,8 @@ static GWAUTHENTICATOR MyObject =
     max_admin_auth_is_client_ssl_capable,       /* Check if client supports SSL  */
     max_admin_auth_authenticate,                /* Authenticate user credentials */
     max_admin_auth_free_client_data,            /* Free the client data held in DCB */
-    users_default_loadusers
+    users_default_loadusers,
+    NULL
 };
 
 /**