Using core Access Control module to limit changing viewers/editors

Issue Summary

First patch contribution for Drupal! I came across the need to allow a post's author to edit the post but not be able to change the Coherent Access settings for that post. Took me five minutes, only added three lines of code, but there's now the ability to control access to Coherent Access settings on a post via the core Access Control page. This is completely role-based, so certain roles can be given ability to change settings.