accesslib: initial has_capability() rewrite

accesslib: initial has_capability() rewrite

 - has_capability() renamed has_capability_old() and we fallback
   when we cannot answer the question

 - has_capability() works great for the course-and-above contexts
   for the logged in user - does not touch the DB

 - works based on $USER->access which has a pretty compact view
   of the user's access rights...

TODO:

 - deal with contexts below the course - here we need to
   trigger the role-assignment and role-defs load when needed

 - deal with "other" contexts that hang from the system context

 - deal with global roleswitch, local roleswitch