I have collected code that used to be in lib/adminlib.php, lib/accesslib.php and user/selector/lib.php into a new admin/roles/lib.php file.
And I added standard GPL and Moodle comments to all the files in admin/roles.
At the same time, I took the opportunity to try to work on some of the usability issues on this page. Note that I have not quite finished! So don't comment until tomorrow.
In the course of doing this, I also did: half of
MDL-11529 Show the number of assignees of each role in the change role dropdown on this page.
MDL-17067 Make it clear in the UI that admins are not allowed to unassign themselves.