MINOR: mux: register the pass-through mux for any ALPN string

The pass-through mux is the fallback used on any incoming connection
unless another mux claims the ALPN name and the proxy mode. Thus mux_pt
registers ALPN token "" (empty name) which catches everything.

src/mux_pt.c

@@ -59,3 +59,13 @@ const struct mux_ops mux_pt_ops = {
 	.wake = mux_pt_wake,
 	.name = "PASS",
 };
+
+/* ALPN selection : default mux has empty name */
+static struct alpn_mux_list alpn_mux_pt =
+	{ .token = IST(""), .mode = ALPN_MODE_ANY, .mux = &mux_pt_ops };
+
+__attribute__((constructor))
+static void __mux_pt_init(void)
+{
+	alpn_register_mux(&alpn_mux_pt);
+}