control: make control links many to many

Output controls can be linked to many input controls and many input
controls can receive input from many output controls. Keep the control
link information inside the link.

src/modules/module-client-node/client-node.c

@@ -792,10 +792,10 @@ impl_node_port_set_io(struct spa_node *node,
 		      uint32_t id,
 		      void *data, size_t size)
 {
-	/* ignore io on the node itself, weonly care about the io on the
+	/* ignore io on the node itself, we only care about the io on the
 	 * port mixers, the io on the node ports itself is handled on the
 	 * client side */
-	return 0;
+	return -EINVAL;
 }
 
 static int