Add pane_pipe_pid with pipe fd and call setpgid to make it easier to kill.

tmux.h

@@ -1213,6 +1213,7 @@ struct window_pane {
 	enum client_theme last_theme;
 
 	int		 pipe_fd;
+	pid_t		 pipe_pid;
 	struct bufferevent *pipe_event;
 	struct window_pane_offset pipe_offset;