forum subscriptions: MDL-14876 user of deprecated functions was breaking managing subscribers.
This was visible when, for example, unusual role definitions meant that someone could access a forum without being enrolled in a course.
Note that one of the places that was previously broken was front page forums. Since subscribers.php does not do paging at all, the fact that I have fixed this bug makes this page dangerous on large sites. A proper solution will have to wait until bug 17550 is fixed in HEAD.
This also fixes a minor problem introduced by MDL-12979.