IAX2 "Ghost" Channels

IAX2 "Ghost" Channels

There is a bug tracker issue where people are reporting "Ghost" channels in their 'iax2 show channels' output.  The confusion is caused by channels being listed as "(NONE)" with format "unknown".  These are not channels of coarse.  They are usually just pending registration or poke requests, but it is confusing output.  To help make sense of this I have added two columns to 'iax2 show channels'.  One shows the first message which started the transaction, and the second shows the last message sent by either side of the call.  This helps diagnose why the entry exists and why it may not go away.

(closes issue #14207)
Reported by: clive18

Review: https://reviewboard.asterisk.org/r/246/

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@194557 65c4cc65-6c06-0410-ace0-fbb531ad65f3