session-helper: use proper fd with ioctl() for tty setup

This was using the "from" fd from the fd_map, which will have already been
closed by time we reach this portion of the child setup.

Tracking the movement of FDs while resolving the remappings is rather
tedious and error prone, so just locate the final fd before calling
the ioctl() in child setup.

session-helper/flatpak-session-helper.c

@@ -142,7 +142,21 @@ child_setup_func (gpointer user_data)
   setpgid (0, 0);
 
   if (data->set_tty)
-    ioctl (data->tty, TIOCSCTTY, 0);
+    {
+      /* data->tty is our from fd which is closed at this point.
+       * so locate the destnation fd and use it for the ioctl.
+       */
+      for (i = 0; i < data->fd_map_len; i++)
+        {
+          if (fd_map[i].from == data->tty)
+            {
+              if (ioctl (fd_map[i].final, TIOCSCTTY, 0) == -1)
+                g_warning ("ioctl(%d, TIOCSCTTY, 0) failed: %s",
+                           fd_map[i].final, strerror (errno));
+              break;
+            }
+        }
+    }
 }