Don't use drupal_not_found as page callback

Problem/Motivation

Currently the special menu items register menu items with drupal_not_found() as page callback. This can lead to errors like: Cannot modify header information - headers already sent by (output started at....
This is due the fact that drupal_not_found() already calls drupal_deliver_page(), which later is called again as this is the delivery callback.

Proposed resolution

As in the documentation of drupal_not_found() defined we should have an own page callback which just returns drupal_not_found instead.

Remaining tasks

Reviews needed.

User interface changes

None.

API changes

None.