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

matthiasclasen/flatpak,matthiasclasen/flatpak,handsome-feng/flatpak,flatpak/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,GeorgesStavracas/flatpak,GeorgesStavracas/flatpak,GeorgesStavracas/flatpak,handsome-feng/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,handsome-feng/flatpak,matthiasclasen/flatpak,flatpak/flatpak,handsome-feng/flatpak,handsome-feng/flatpak,GeorgesStavracas/flatpak,GeorgesStavracas/flatpak