When usernames are numeric, they are lost in mnet transit when used as
keys in the array result of course_enrolments(). As the username is not
sent with the enrolment details, it was not previously recoverable.
This patch adds the information to the enrolment details,
(enrol/mnet/enrol.php), and adds support for recovering the username if
it was numeric (and hence lost) (admin/mnet/enr_course_enrol.php)