When specifying a gateway (-g), add a route appropriate to the address
family of the destination.

Reference: /n/sources/patch/applied/ipconfig-ipv6-gateway
Date: Fri Oct 13 21:29:14 CES 2006
Signed-off-by: lyndon@orthanc.ca

--- /sys/src/cmd/ip/ipconfig.c	Fri Oct 13 21:27:04 2006
+++ /sys/src/cmd/ip/ipconfig.c	Fri Oct 13 21:27:00 2006
@@ -564,7 +564,11 @@
 	cfd = open(buf, ORDWR);
 	if(cfd < 0)
 		return;
-	fprint(cfd, "add 0 0 %I", gaddr);
+	if(isv4(gaddr)) {
+		fprint(cfd, "add 0 0 %I", gaddr);
+	} else {
+		fprint(cfd, "add :: :: %I", gaddr);
+	}
 	close(cfd);
 }