accesslib: default, guest and notloggedin roles work properly now

accesslib: default, guest and notloggedin roles work properly now

Changes around load_user_capability() and has_capability() to make
the default role fallbacks and guest/nonloggedin roles work.

This commit also introduces the concept of having a magic
context next to the root context in $USER->access[ra], as

 $USER->access[ra][/1] = 1 (admin roleid)
 $USER->access[ra][/1:def] = 7 (loggedinuser roleid)

and has_cap_fromsess() now checks for that magic context
as well.