MINOR: listener: move bind_proc and bind_thread to struct settings

As mentioned previously, these two fields come under the settings
struct since they'll be used to bind receivers as well.
diff --git a/src/proto_tcp.c b/src/proto_tcp.c
index 56e9391..57bef86 100644
--- a/src/proto_tcp.c
+++ b/src/proto_tcp.c
@@ -768,7 +768,7 @@
 	listener->state = LI_LISTEN;
 
 	fd_insert(fd, listener, listener->proto->accept,
-	          thread_mask(listener->bind_conf->bind_thread) & all_threads_mask);
+	          thread_mask(listener->bind_conf->settings.bind_thread) & all_threads_mask);
 
 	/* for now, all regularly bound TCP listeners are exportable */
 	if (!(listener->options & LI_O_INHERITED))