Add a generic function to set the bridged call PVT unique id string

as a channel variable BRIDGEPVTCALLID

This is important for call tracing in log files and CDRs, so that
the SIP callID can be traced along servers.

The CHANNEL dialplan function won't work here, since the outbound
channel is gone when we need the Call-ID.

Other channel drivers may now implement the same function :-),
but this patch only supports chan_sip.so.

Inspired by (issue #11816)
Reported by: ctooley

Patch by oej



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@99644 65c4cc65-6c06-0410-ace0-fbb531ad65f3