New --ipv6 option and fix learning of external IPv6 from other peers

When re6st attempts to use UPnP and IPv6 is enabled at the same time,
the external IPv4 was published for IPv6 protocols.
For example, machine6 in the demo had:
  10.0.1.3,1194,tcp;10.0.1.3,1194,udp;10.0.1.3,1195,udp6

This caused re6stnet to crash (socket.gaierror) if GEOIP2_MMDB is set.

With this commit, IPv4 & IPv6 are now processed independently.

demo/m6/re6stnet.conf

 log m6/
 run m6/run
 state m6/
+ipv6 fc42:6::1
 pp 1194 tcp
 pp 1194 udp
 pp 1195 udp6

re6st/cli/node.py

@@ -23,6 +23,9 @@ def getConfig():
              "- any: ask peers our IP\n"
              " (default: like 'upnp' if miniupnpc is installed,\n"
              "  otherwise like 'any')")
+    _('--ipv6', action='append', default=[],
+        help="IPv6 address advertised to other nodes."
+             " If not given, ask peers our IP.")
     _('--registry', metavar='URL', required=True,
         help="Public HTTP URL of the registry, for bootstrapping.")
     _('-l', '--log', default='/var/log/re6stnet',
@@ -166,7 +169,7 @@ def main():
         # Make sure we won't tunnel over re6st.
         config.disable_proto = tuple(set(('tcp6', 'udp6')).union(
             config.disable_proto))
-    address = ()
+    address = []
     server_tunnels = {}
     forwarder = None
     if config.client:
@@ -181,18 +184,23 @@ def main():
         else:
             pp = [x for x in ((1194, 'udp'), (1194, 'tcp'))
                     if x[1] not in config.disable_proto]
+        ipv4_any = []
+        ipv6_any = []
+        for x in pp:
+            server_tunnels.setdefault('re6stnet-' + x[1], x)
+            (ipv4_any if x[1] in ('tcp', 'udp') else ipv6_any).append(x)
         def ip_changed(ip):
-            for family, proto_list in ((socket.AF_INET, ('tcp', 'udp')),
-                                       (socket.AF_INET6, ('tcp6', 'udp6'))):
             try:
-                    socket.inet_pton(family, ip)
-                    break
+                socket.inet_aton(ip)
             except socket.error:
-                    pass
+                family = socket.AF_INET6
+                pp = ipv6_any
             else:
-                family = None
-            return family, [(ip, str(port), proto) for port, proto in pp
-                            if not family or proto in proto_list]
+                if forwarder:
+                    return forwarder.checkExternalIp(ip)
+                family = socket.AF_INET
+                pp = ipv4_any
+            return family, [(ip, str(port), proto) for port, proto in pp]
         if config.gw_list:
           gw_list = deque(config.gw_list)
           def remote_gateway(dest):
@@ -220,15 +228,15 @@ def main():
                 logging.info("%s: assume we are not NATed", e)
             else:
                 atexit.register(forwarder.clear)
-                for port, proto in pp:
+                for port, proto in ipv4_any:
                     forwarder.addRule(port, proto)
-                ip_changed = forwarder.checkExternalIp
-                address = ip_changed(),
+                address.append(forwarder.checkExternalIp())
         elif 'any' not in config.ip:
-            address = map(ip_changed, config.ip)
-            ip_changed = None
-        for x in pp:
-            server_tunnels.setdefault('re6stnet-' + x[1], x)
+            address += map(ip_changed, config.ip)
+            ipv4_any = ()
+        if config.ipv6:
+            address += map(ip_changed, config.ipv6)
+            ipv6_any = ()
     else:
         ip_changed = remote_gateway = None