bulk enrolment actions fail when multiple instances of plugin present

Bulk enrolment actions (e.g. initiated from user/index.php) fail when multiple instances of the same plugin are present for a course. In our case, we have several different self-enrolment possibilities, and attempts to bulk-unenrol fail.

The cause is that the "plugin" parameter being passed to enrol/action-redir.php is inadequate to describe the instance, and action-redir.php identifies and obtains a manager object for the wrong one, subsequently finding no matching userids and removing the specified users from the list upon which it should act.