Fixed version of the code.

This really belongs as a patch to the UC roles module IMO.

I've also:

- removed $polarity_widget form element as it's not used
- removed the 'Add role' -- as far as I can see this is just a kludge to get uc_roles_user_submit() to act.
- changed the form to allow changing / assigning more than one role at a time
- added help text and an explanatory note in the event that there are no available roles.