Add a "void *if_carp" placeholder to struct ifnet with prospect to bring in

the "Common address redundancy protocol" (CARP) during the 5-STABLE cycle.
Hence doing the ABI break now.

Approved by:	re (scottl)

sys/net/if_var.h

@@ -148,6 +148,7 @@ struct ifnet {
 		 */
 	struct	klist if_klist;		/* events attached to this if */
 	int	if_pcount;		/* number of promiscuous listeners */
+	void	*if_carp;		/* carp (tbd) interface pointer */
 	struct	bpf_if *if_bpf;		/* packet filter structure */
 	u_short	if_index;		/* numeric abbreviation for this if  */
 	short	if_timer;		/* time 'til if_watchdog called */