Allow more complex scenarios in execute_hook_menu() by passing $callbacks as reference

Complex scenario used here: Admin Views.

Since admin_views replaces system paths it has to make sure the child items are set up properly. But this only works if the $callbacks are passed by reference since the items are "locked" in the latest views version.

A more detailed description can be found here #1177202-14: admin_views module needs to be updated to work with views 7.x-3.0-beta3

The attached patch simply makes $callbacks a reference.