Sync OpenBSD patchset 1069:

Add notify hooks for various events, the functions are currently empty
stubs but will be filled in for control mode later. From George Nachman.

notify.c

@@ -0,0 +1,59 @@
+/* $Id$ */
+
+/*
+ * Copyright (c) 2012 George Nachman <tmux@georgester.com>
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF MIND, USE, DATA OR PROFITS, WHETHER
+ * IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING
+ * OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+#include "tmux.h"
+
+void
+notify_window_layout_changed(unused struct window *w)
+{
+}
+
+void
+notify_window_unlinked(unused struct session *s, unused struct window *w)
+{
+}
+
+void
+notify_window_linked(unused struct session *s, unused struct window *w)
+{
+}
+
+void
+notify_window_renamed(unused struct window *w)
+{
+}
+
+void
+notify_attached_session_changed(unused struct client *c)
+{
+}
+
+void
+notify_session_renamed(unused struct session *s)
+{
+}
+
+void
+notify_session_created(unused struct session *s)
+{
+}
+
+void
+notify_session_closed(unused struct session *s)
+{
+}