lib/nat, lib/connections: Fix a few issues with NAT traversal

1. For the same internal port we ask for the same external port on all devices. This can be a problem if one device speaks over two protocols.
2. Always add a nil address even if we managed to get external address of the gateway, just because the gateway might be in DMZ behind another gateway.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3196

lib/connections/tcp_listen.go

@@ -137,6 +137,15 @@ func (t *tcpListener) WANAddresses() []*url.URL {
 			// Does net.JoinHostPort internally
 			uri.Host = addr.String()
 			uris = append(uris, &uri)
+
+			// For every address with a specified IP, add one without an IP,
+			// just in case the specified IP is still internal (router behind DMZ).
+			if len(addr.IP) != 0 && !addr.IP.IsUnspecified() {
+				uri = *t.uri
+				addr.IP = nil
+				uri.Host = addr.String()
+				uris = append(uris, &uri)
+			}
 		}
 	}
 	t.mut.RUnlock()