Add a way (refresh-client -r) for control mode clients to provide OSC 10

and 11 responses to tmux so they can set the default foreground and
background colours, from George Nachman in GitHub issue 4014.

window.c

@@ -937,6 +937,9 @@ window_pane_create(struct window *w, u_int sx, u_int sy, u_int hlimit)
 
 	wp->pipe_fd = -1;
 
+	wp->control_bg = -1;
+	wp->control_fg = -1;
+
 	colour_palette_init(&wp->palette);
 	colour_palette_from_option(&wp->palette, wp->options);