Currently there is no easy way for ESL and dial plan users to easily correlate
the channel UUID of the call legs involved in a nightmare transfer. This patch
adds two new channel variables, which are set to the remote server call leg
UUIDs (transfer_refer_from, transfer_refer_for).

The UUIDs are passed from the remote server in custom headers (X-FS-Refer-From
and X-FS-Refer-For).