Open /dev/ptm before pledge() and save it to be used for PTMGET later

(this means inlining forkpty()).

ok deraadt

window.c

@@ -886,7 +886,7 @@ window_pane_spawn(struct window_pane *wp, int argc, char **argv,
 	ws.ws_col = screen_size_x(&wp->base);
 	ws.ws_row = screen_size_y(&wp->base);
 
-	switch (wp->pid = forkpty(&wp->fd, wp->tty, NULL, &ws)) {
+	switch (wp->pid = pty_fork(ptm_fd, &wp->fd, wp->tty, sizeof wp->tty, &ws)) {
 	case -1:
 		wp->fd = -1;
 		xasprintf(cause, "%s: %s", cmd, strerror(errno));