refactor of sip_i_invite.  Much more error handling of different missing data on the inbound invite.  Some minor changes in behavior.  Chanvars are now not set for vars we don't have.  the channel name will now include :port (the full uri as we create it from the data available), display name is now unknown not service if there is not one available.  Added contact uri as a chan var like all the others, the channel name if there is no user portion of the from will not have a from part of service, it will have no user part or @.  The contact will now be used for the channel name if there is no from header in the invite.

git-svn-id: http://svn.freeswitch.org/svn/freeswitch/trunk@4388 d0543943-73ff-0310-b7d9-9358b9ac24b2