Simple patch to make it easier to call drupal_execute()

From the Drupal Docs:

In addition, developers using submit functions to persist data to the database are encouraged to return meaningful data about the results of the operation (ids of newly created nodes, etc.) in the $form_state. This allows developers calling forms using drupal_execute() to properly handle to the results of a form's successful submission.

I've attached a very small and simple patch to help do that for the Custom Pages section. The new page id should be passed on, successfully, to the next piece of code seeing the $form_state.

Since it's so simple and (I'd think) non-controversial, I'm marking as RTBC.