Make pane scrollbars able to auto-hide after a short timeout. This

replaces the previous "modal" behaviour where the pane would be resized,
which tended to make a mess. Instead, the scrollbar appears when
scrolling or when hovered over with the mouse and disappears a
(configurable) short period later. From Michael Grant.

regsub.c

@@ -24,7 +24,8 @@
 #include "tmux.h"
 
 static void
-regsub_copy(char **buf, ssize_t *len, const char *text, size_t start, size_t end)
+regsub_copy(char **buf, ssize_t *len, const char *text, size_t start,
+    size_t end)
 {
 	size_t	add = end - start;