Sync OpenBSD patchset 1084:

Add a layout history which can be stepped through with select-layout -u
and -U commands (bound to 'u' and 'U' by default).

layout.c

@@ -19,6 +19,7 @@
 #include <sys/types.h>
 
 #include <stdlib.h>
+#include <string.h>
 
 #include "tmux.h"
 
@@ -745,3 +746,74 @@ layout_close_pane(struct window_pane *wp)
 	}
 	notify_window_layout_changed(wp->window);
 }
+
+/* Add layout to list. */
+void
+layout_list_add(struct window *w)
+{
+	struct last_layout	*ll, *ll_last;
+	char			*layout;
+	u_int			 limit;
+
+	layout = layout_dump(w);
+
+	ll_last = w->layout_list_last;
+	if (ll_last != NULL && strcmp(ll_last->layout, layout) == 0) {
+		free(layout);
+		return;
+	}
+
+	ll = xmalloc(sizeof *ll);
+	ll->layout = layout;
+	if (ll_last == NULL)
+		TAILQ_INSERT_TAIL(&w->layout_list, ll, entry);
+	else
+		TAILQ_INSERT_AFTER(&w->layout_list, ll_last, ll, entry);
+	w->layout_list_size++;
+	w->layout_list_last = ll;
+
+	limit = options_get_number(&w->options, "layout-history");
+	while (w->layout_list_size > limit) {
+		ll = TAILQ_LAST(&w->layout_list, last_layouts);
+		if (ll == w->layout_list_last)
+			ll = TAILQ_FIRST(&w->layout_list);
+
+		TAILQ_REMOVE(&w->layout_list, ll, entry);
+		w->layout_list_size--;
+
+		xfree(ll->layout);
+		xfree(ll);
+	}
+}
+
+/* Apply next layout from list. */
+const char *
+layout_list_redo(struct window *w)
+{
+	struct last_layout	*ll, *ll_last;
+
+	ll_last = w->layout_list_last;
+	if (ll_last == NULL)
+		return (NULL);
+	ll = TAILQ_NEXT(ll_last, entry);
+	if (ll == NULL)
+		return (NULL);
+	w->layout_list_last = ll;
+	return (ll->layout);
+}
+
+/* Apply previous layout from list. */
+const char *
+layout_list_undo(struct window *w)
+{
+	struct last_layout	*ll, *ll_last;
+
+	ll_last = w->layout_list_last;
+	if (ll_last == NULL)
+		return (NULL);
+	ll = TAILQ_PREV(ll_last, last_layouts, entry);
+	if (ll == NULL)
+		return (NULL);
+	w->layout_list_last = ll;
+	return (ll->layout);
+}