In a very weird situation, our declaration of `shutdown()` shadows a
function `shutdown(int, int)` from sys/socket, which _just happens_ to
be called by libxcb when closing the sdl window. This calls
`main_clean_exit` which calls into micropython and causes at best an
uncaught NLR and at worst an outright segfault because by that time the
micropython environment doesn't exist anymore.
I didn't think this sort of thing would be possible but here we are??
Fixed by removing `__shutdown()` and replacing `shutdown` with
`trezor_shutdown`
