stf: merge the missing half of the stf implementation

Thumbs up to the pfSense guys for publishing this!  STF (6rd) patching
is probably going away for us with 11-CURRENT though, unless more work
is done in FreeBSD.  Contacted the original maintainer a while ago, but
no response.

Other BSDs are in a similar state, suggesting that GIF basically works.

Taken from: pfsense/FreeBSD-src@73261ef4d5a
See also:   https://lists.freebsd.org/pipermail/freebsd-net/2013-June/035749.html
See also:   http://undeadly.org/cgi?action=article&sid=20130828151241

sbin/ifconfig/ifstf.c

@@ -73,8 +73,8 @@ stf_status(int s)
 	if (do_cmd(s, STF_GV4NET, &param, sizeof(param), 0) < 0)
 		return;
 
-	printf("\tv4net %s/%d\n", inet_ntoa(param.inaddr), param.prefix);
-	printf("\tv4br %s\n", inet_ntoa(param.dstv4_addr));
+	printf("\tv4net %s/%d -> ", inet_ntoa(param.inaddr), param.prefix ? param.prefix : 32);
+	printf("tv4br %s\n", inet_ntoa(param.dstv4_addr));
 
 	return;
 }