are taken down by the C++ runtime environment. This will later be done
with runlevels. Why would we want to run nasal code in subsystem
destructors? We don't really. But some data structures may use nasal,
which are normally created/destroyed during runtime. And these will
also be destroyed at fgfs exit. In the past things like these didn't
happen, because someone had disabled all subsystem destructors ...
explicit calls to shutdown_all() which was causing this to be called twice.
This could cause problems with some IO modules (such as attempting to close
an invalid file descriptor the second time.)
