#1186208 by mrfelton, chx: Fix callback inheritance by removing the empty(['page callback']). (The actual fix is in the prior commit.)

Remove the empty(['page callback']). Core inherits from parent and if there is no page callback not much happens on the page anyways.

#1186208 by chx, salvis, mrfelton: Improve the stability and throw an exception for MENU_DEFAULT_LOCAL_TASKs.

Issue #1079032 by salvis: Create test infrastructure and a first test to check the ability to provide access to admin/people/create.

An API module to help client modules chain their access callbacks into other modules' menu items. Initial version written by chx in 3.5 minutes.