Store call CRV in caller_data.call_reference and use call private to hold the
FreeTDM channel object.

Remove isdn_data->channels_{local,remote,outbound}_crv arrays.

Allow (and force) inbound call state transition DIALTONE -> DOWN for
incoming RELEASE COMPLETE messages in NT mode.

Dialtone in NT mode works, everything else needs more testing.
Signed-off-by: Stefan Knoblich <stkn@openisdn.net>