Don't allow users to select role more than once, even from account page

Issue Summary

Hello!

In our setup, we recently had a need to block users from modifying which user-selectable role they'd chosen once they'd already chosen one. Ordinarily this would be handled by turning off the "Registration and user account forms" setting, but since we're using LDAP integration for auth, we don't use the normal Drupal registration process. As a result, I've rolled the attached patch, which adds one more option to the UI…"User can select roles more than once", which to preserve BC, is true by default.

If this new option is unchecked, users will not be able to switch to a different user-selectable role after they've chosen one, regardless of whether they chose it via registration or account editing.

Hope this helps someone!

Thanks,
Adam