Convert admin/mnet/enr_course_enrol.php/html to use the new user_selector class

This is equivalent change to admin/roles/assign.php: http://cvs.moodle.org/moodle/admin/roles/assign.php?r1=1.84&r2=1.85, done as subtask MDL-16993 of MDL-16966.

Note that the same commit (second commit on http://tracker.moodle.org/browse/MDL-16993?page=com.atlassian.jira.plugin.system.issuetabpanels:cvs-tabpanel) also eliminated admin/roles/assign.html, and the SQL and calls to accesslib.php methods removed from admin/roles/assign.php ended up in user/selector/lib.php: http://cvs.moodle.org/moodle/user/selector/lib.php?r1=1.7&r2=1.8

I'm afraid that commit also contains some noise, because I was still refining the user_selector API at the time.

I think it would be really good if someone other than me did this. It would be a good test of my user_selector API. However, if no-one else has done this before the 2.0 release, I don't mind this being assigned back to me. Well, don't mind much.