This is basically a complete rollback of r155401, as it was determined that

it would be best to maintain API compatibility.  Instead, this commit introduces
ao2_callback_data() which is functionally identical to ao2_callback() except
that it allows you to pass arbitrary data to the callback.

Reviewed by Mark Michelson via ReviewBoard:
	http://reviewboard.digium.com/r/64


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@158959 65c4cc65-6c06-0410-ace0-fbb531ad65f3