get_my_courses() and get_user_courses_bycap() field handling and caching

get_my_courses() and get_user_courses_bycap() field handling and caching

- Field handling moves back to get_my_courses() and now we have
  almost all the fields that the old get_my_courses() did
  (except for summary, which is *huge*) so get_my_courses() asks
  for a lot of fields, but the get_user_courses_bycap() defaults
  are _much_ leaner now.

  I think this makes sense ;-)

- get_my_courses() now caches the course ids for the currently logged in
  user in $USER->mycourses -- as a _string_. This is magnitudes more efficient
  than having it as an array.

The cache makes a difference, but it's not very visible on
normal pageloads (with my courses block, for example).

However, over 100 iterations, for a user with 50 enrolments in a site
with 6K courses, we go from 4.3s to 0.6s. And the DB queries are *cheap*.

            $tt = microtime(true);
            for($n=0;$n<100;$n++) {
                get_my_courses($USER->id, 'sortorder ASC');
            }
            error_log("took " .  (microtime(true) - $tt));