accesslib - role_switch() revamp, introduce get_role_access_bycontext()

accesslib - role_switch() revamp, introduce get_role_access_bycontext()

First stage of role_switch() rewrite - role_switch() sets up
a rsw entry in $USER->access, and makes sure we have the
appropriate role definitions in rdef.

That is where get_role_access_bycontext() comes into play -
gets all the rdef entries in one cheap sql query.

... though there may be many of them...

TODO:

 - fix callers of role_switch()
 - teach has_cap_fromsess() to read the rsw entries