(1) after removing a shutdown hook (in shutdownhook_disestablish()),
free it. We created it, we have to free it. Without this,
shutdownhook_disestablish() leaks memory.
(2) in doshutdownhooks(), before running each hook, remove it from the
shutdown hook list. This makes sure that every hook is tried once
(because doshutdownhooks() is called from before rebooting, and
a fault in a shutdown hook will cause doshutdownhooks() to be called
again), but prevents the hooks from potentially being run infinitely
(as used to be possible, in the above-mentioned situation).
