Allow CiviMail "unsubscribe group" to be non-Public

In 4.6.3, if I search for contacts and use the action "schedule mass mailing" on the results, I'm required to specify an "unsubscribe group".

The problem is that the screen restricts me to choose a Public list.

This is asymmetric with the Send Mailing functionality, where I can send to a non-Public list, and the recipient can unsubscribe from that non-Public list.

I have made a one-line change that allows any active group to be chosen as the Unsubscribe Group, which I will supply as a PR. I've tested that the recipient of such a mailing can unsubscribe from the non-Public list.