Change name of "contact" setting to "callback" which better reflects what it

is to the person that configures asterisk. That we use it internally in the
contact header is a totally different story.

Still not convinced this is a good option.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@46489 65c4cc65-6c06-0410-ace0-fbb531ad65f3