Run the process session in a test executable linked with libc and leave
through LibcExit and LibcExitWithoutHandlers: the backend is released once
in both. Check that a refused request is never released, that off never
reaches the backend, that a second session is refused, that an unset
APS5_GAME_MODE is accepted and that APS5_GAME_MODE=bogus is rejected with
an error naming the variable.
