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/common/sender.h

@@ -14,9 +14,6 @@ typedef struct sender {
 	struct sender		*slave;			/* points to audio device slave member */
 	struct sender		*master;		/* points to audio device master source */
 
-	/* call reference */
-	int			callref;
-
 	/* system info */
 	int			kanal;			/* channel number */