Support listening for IPv6 connections

The EchoLink proxy protocol supports only IPv4. However, the connection
between the proxy and the EchoLink client doesn't need to be IPv4, so
listening for incoming clients on IPv6 makes sense. It makes even more
sense moving forward because proxies like this may become the only
option for accessing EchoLink from IPv6-only hosts.

Author: cottsay

include/conn.h

@@ -217,6 +217,6 @@ void conn_shutdown(struct conn_handle *conn);
  * @param[in] conn Target network connection instance
  * @param[out] dest Destination ASCII string
  */
-void conn_get_remote_addr(const struct conn_handle *conn, char dest[40]);
+void conn_get_remote_addr(const struct conn_handle *conn, char dest[46]);
 
 #endif /* _conn_h */

src/conn.c

@@ -226,7 +226,7 @@ int conn_listen(struct conn_handle *conn)
 		return -1;
 	}
 
-	hints.ai_family = AF_INET;
+	hints.ai_family = AF_UNSPEC;
 	hints.ai_flags = AI_PASSIVE;
 
 	ret = getaddrinfo(conn->source_addr, conn->source_port == NULL ? "0" : conn->source_port, &hints, &res);
@@ -403,7 +403,7 @@ int conn_connect(struct conn_handle *conn, const char *addr, const char *port)
 		goto conn_connect_free_early;
 	}
 
-	priv->sock_fd = socket(AF_INET, res->ai_socktype, res->ai_protocol);
+	priv->sock_fd = socket(res->ai_family, res->ai_socktype, res->ai_protocol);
 	if (priv->sock_fd == INVALID_SOCKET)
 	{
 		ret = SOCK_ERRNO;
@@ -782,11 +782,24 @@ void conn_shutdown(struct conn_handle *conn)
 	mutex_unlock_shared(&priv->mutex);
 }
 
-void conn_get_remote_addr(const struct conn_handle *conn, char dest[40])
+void conn_get_remote_addr(const struct conn_handle *conn, char dest[46])
 {
-	struct conn_priv *priv = (struct conn_priv *)conn->priv;
-	struct sockaddr_in *addr = (struct sockaddr_in *)&priv->remote_addr;
-	inet_ntop(addr->sin_family, &addr->sin_addr.s_addr, dest, 40);
+	const struct conn_priv *priv = (const struct conn_priv *)conn->priv;
+	const struct sockaddr_in *addr = (const struct sockaddr_in *)&priv->remote_addr;
+	const struct sockaddr_in6 *addr6 = (const struct sockaddr_in6 *)&priv->remote_addr;
+
+	switch (priv->remote_addr.ss_family)
+	{
+	case AF_INET:
+		inet_ntop(addr->sin_family, &addr->sin_addr.s_addr, dest, 46);
+		break;
+	case AF_INET6:
+		inet_ntop(addr6->sin6_family, &addr6->sin6_addr.s6_addr, dest, 46);
+		break;
+	default:
+		strcpy(dest, "(unknown)");
+		break;
+	}
 }
 
 int conn_in_use(struct conn_handle *conn)