Add intf_name to the route_entry struct.

This is set to an empty string in all functions yielding routes,
particularly route_loop. The code to get the interface pertaining to a
route is different on different platforms, so must be added one by one.
The code setting the intf_name to an empty string is only tested on
Linux.

libdnet-stripped/include/dnet/route.h

@@ -15,6 +15,7 @@
  * Routing table entry
  */
 struct route_entry {
+	char		intf_name[INTF_NAME_LEN];	/* interface name */
 	struct addr	route_dst;	/* destination address */
 	struct addr	route_gw;	/* gateway address */
 };