MXS-1354: Add creation of basic/admin users to maxctrl

The type of the created user can now be specified with the --type option.

Expanded tests that cover the user creation. Also added a test case that
checks that basic users are only allowed to read through the REST API.

server/core/admin.cc

@@ -226,7 +226,7 @@ int handle_client(void *cls,
 {
     if (!do_auth(connection, url, method))
     {
-        return MHD_YES;
+        return MHD_NO;
     }
 
     if (*con_cls == NULL)