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/anetz/anetz.h

@@ -19,6 +19,7 @@ typedef struct anetz {
 
 	/* sender's states */
 	enum anetz_state	state;			/* current sender's state */
+	int			callref;		/* call reference */
 	char			station_id[8];		/* current station ID */
 	struct timer		timer;