accesslib: Intro global $ACCESS and load_user_accessdata()

accesslib: Intro global $ACCESS and load_user_accessdata()

This patch introduces a new global $ACCESS that will cache
accessdata (our ra/rdef array) for users that are _not_ the
logged in user.

Most of the time it will be empty (luckily).

Each new user we have to get "in" costs us 3 or 4 cheap
dbqueries at the moment, so it is not that bad.

has_capability() now calls load_user_accessdata() if the
call is for a non-logged-user. So $ACCESS is autopopulated
transparently.

This also means that has_capability no longer calls
has_capability_old(). Yay!

With this patch, user/view.php for looking at a user in a course...

  Before: 62 dbqueries, 10.3MB, 0.7s
  After:  49 dbqueries,  8.5MB, 0.3s