Only allow certain roles to modify private setting

Issue Summary

Scenario:

I want the vast majority of uploads on my site to be private. I still want my role to have the ability to mark some uploads as public, but I want the day-to-day editors of the site to always make uploads private. Preferably by disabling the private checkbox, so that they can *see* whether the upload is private, but not (without html/javascript hacking) change its setting.

Proposed Solution:

See attached patch. By default, my patch does nothing to the behavior of Private Upload (I think!). It adds a field to the Private Upload Settings form called "Control Access to Private Checkbox". If this option is checked, then an "access private checkbox" permission is added to admin permissions page. For roles that do not have this permission, my patch disables the private checkbox (just in HTML -- not enforced server-side).

I'm already using this patch on my site, and am only sharing this patch to contribute up-stream. If it's of use to you or you want to incorporate it into the module, great! Critiques welcome.