#42826 by jvandyk, mfb, macgirvin. Only show 'edit primary links' link if the user has access to it.
backport from HEAD

Backport from head.

fixes 57406 too

patch by chx

  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.

  Deprecates the primary_links module.

  This patch was much needed.  Thanks Richards!

  NOTE: if some themers could investigate if there is room for improvement
        with regard to theming, that would be awesome.