Add a flag to move-window to renumber the windows in a session (closing

any gaps) and add an option to do this automatically each time a window
is killed. From Thomas Adam.

server-fn.c

@@ -263,6 +263,9 @@ server_kill_window(struct window *w)
 			} else
 				server_redraw_session_group(s);
 		}
+
+		if (options_get_number(&s->options, "renumber-windows"))
+			session_renumber_windows(s);
 	}
 }