refactor coherent access options display

Issue Summary

imo, this module would be more generalized if the 'Shared Editing' collapsing group (with 'add viewers' and 'add editors' options on the node form) was instead configurable and would work whether I wanted to add editors or viewers or both.

I'm trying to make a 'Message' node type have the "Add Viewers" display, but change the language to "To:" without any help text. And I would like to be able to uncheck 'add editors' display. This would be a very elegant solution for private messaging, while no other good one exists for D6.