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/cnetz/cnetz.h

@@ -70,6 +70,7 @@ typedef struct cnetz {
 
 	/* all cnetz states */
 	enum cnetz_state	state;			/* main state of sender */
+	int			callref;		/* call reference while in SpK mode */
 
 	/* cell nr selection */
 	int			cell_auto;		/* if set, cell_nr is selected automatically */