Don't force IPv6 if IPv6 name servers

Just because we may have found IPv6 DNS servers from /etc/resolv.conf
doesn't mean we should force IPv6 support.

Instead we should disable the IPv6 DNS servers if IPv6 is disabled.

Note: this will also result in any IPv6 servers specified on the command
line being silently ignored too.

Specifying an IPv6 subnet will still require IPv6 support.

Closes #74

sshuttle/client.py

@@ -547,11 +547,15 @@ def main(listenip_v6, listenip_v4,
         else:
             listenip_v6 = None
 
-    required.ipv6 = len(subnets_v6) > 0 or len(nslist_v6) > 0 \
-        or listenip_v6 is not None
+    required.ipv6 = len(subnets_v6) > 0 or listenip_v6 is not None
     required.udp = avail.udp
     required.dns = len(nslist) > 0
 
+    # if IPv6 not supported, ignore IPv6 DNS servers
+    if not required.ipv6:
+        nslist_v6 = []
+        nslist = nslist_v4
+
     fw.method.assert_features(required)
 
     if required.ipv6 and listenip_v6 is None: