socket-default: Use IPv6-only mode for IPv6 sockets

Otherwise, we can't open a dedicated IPv4 socket on the same port as the
IPv6 socket already is set up do receive IPv4 packets (unless we'd again
enable SO_REUSEADDR).

Fixes: 83da133712 ("socket-default: Don't set SO_REUSEADDR on IKE sockets anymore")

src/libcharon/plugins/socket_default/socket_default_socket.c

@@ -699,6 +699,14 @@ static int open_socket(private_socket_default_socket_t *this,
 		return -1;
 	}
 
+	if (family == AF_INET6 &&
+		setsockopt(skt, SOL_IPV6, IPV6_V6ONLY, &on, sizeof(on)) < 0)
+	{
+		DBG1(DBG_NET, "unable to set IPV6_V6ONLY on socket: %s", strerror(errno));
+		close(skt);
+		return -1;
+	}
+
 	/* bind the socket */
 	if (bind(skt, &addr.sockaddr, addrlen) < 0)
 	{