callref: Move callref from 'sender' to individual implementation of a, b, c-netz

Cellular networks use transaction instances, so callref is now member of
transaction object.

src/bnetz/bnetz.h

@@ -59,6 +59,7 @@ typedef struct bnetz {
 
 	/* all bnetz states */
 	enum bnetz_state	state;			/* main state of sender */
+	int			callref;		/* call reference */
 	enum dial_mode		dial_mode;		/* sub state while dialing is received */
 	enum dial_type		dial_type;		/* defines if mobile supports metering pulses */
 	char			dial_number[14];	/* dial string received */