1) This commit adds the ability to override the To: header so you
   can place an outbound SIP call to a paticular destination.

   For instance, if you have a user registered to your machine as 'gateway' and
   you want to call another extension at that machine 'foo' you can now call:

  sofia/mydomain.com/gateway:foo@anydomain.com

  or if you just want to call some SIP url and put an alternate To:
  so you can do forwarding:

  sofia/mydomain.com/1000@somwhere.com:12122551234@realdest.com


2) You can add headers to outbound SIP calls by
   prefixing the string 'SIP_h_' to any channel variable:

   <action application="set" data="SIP_h_X-Answer=42"/>
   <action application="bridge" data="sofia/mydomain.com/1000@somehost.com"/>

3) All inbound SIP calls will install any X- headers into local variables
   this means in conjunction with item 2, all X- headers will pass across 
   bridged calls auto-magicly.

4) The switch_ivr_transfer_variable function in switch_ivr.c has been modified so if the last arguement
   'var' starts with a ~ it will copy anything that is prefixed with that string rather than an exact match.

*NOTE* you are not required to put an X- prefix in the SIP_h_ vars but 
it is reccommended because most SIP devices will ignore any unknown headers that don't.




git-svn-id: http://svn.freeswitch.org/svn/freeswitch/trunk@4216 d0543943-73ff-0310-b7d9-9358b9ac24b2