Show permissions granted by other roles (which user_permissions can't override)

The attached patch does two big things.

1) Checks the boxes on the User Permissions page granted to a user by roles that user_permissions does not manage
2) Disables them

This makes it clear which permissions can/cannot be managed from that page. I believe there are several issues in the queue here related to people trying to revoke access to something for a user using this page, and confused by the fact that it's a different role granting the permission that user.

It also does a third minor one - On admin/people, it replaces the string "_user_role_$uid" with "(custom permissions)". This is a minor user-interface change to finish hiding the custom roles user_permissions generates, while still letting the admin know that a user's access is customized.