MDL-6041 - Proper fix that eliminates the magic number 99999 when getting lists of...

MDL-6041 - Proper fix that eliminates the magic number 99999 when getting lists of studnets. Now, there is no arbitrary upper limit in the datalib functions, and sensible upper limits on pages that display lists of users. However, if you try to get all the site students, then get_students prints a warning in debug mode, telling you that you need to rethink your code.

Also a few more ISNULL()s eliminated.

And a typo role_assignment -> role_assignments.