Add metric to struct route_entry.

Actually getting a nonzero value for the metric is supported only on
Linux and Windows.

libdnet-stripped/include/dnet/route.h

@@ -18,6 +18,7 @@ struct route_entry {
 	char		intf_name[INTF_NAME_LEN];	/* interface name */
 	struct addr	route_dst;	/* destination address */
 	struct addr	route_gw;	/* gateway address */
+	int		metric;		/* per-route metric */
 };
 
 typedef struct route_handle route_t;