Moves callbacks and arguments to a separate data structure from the visible menu tree. As mentioned on drupal-devel, this change has little impact except to slightly improve code legibility and prepare the code base for future separation of these tasks, with an eye toward performance improvements of the menu system.  Also changes array_key_exists() to isset() in several places, which is reported to perform better.

- Patch #32665 by Richard Archer: add more functionality to menu item settings on node/*/edit pages.

- Patch #38849 by wtanaka: forum icon does not display as locked (aka closed) if comments are "disabled" instead of "read only".

- Patch #1482 by djun: we're moving the DocBook output to a contributed module.  It's not a core feature.

- Patch #38841 by Morbus/Ber: improved documentation about $base_url and rolled back auto-detection.