* libguile/finalizers.c: New excitement! We'll be running finalizers in
threads, if that's available. If it's not available, during early
boot, we can run finalizers in asyncs. This will make it safer to
allocate while holding a mutex.
* libguile/posix.c (scm_fork): Shut down the finalizer thread before
forking.
* libguile/init.c (scm_i_init_guile): Init the async finalizer mechanism
during boot and, if available, initialialize the finalizer thread at
the very end.
* libguile/gc.c (scm_storage_prehistory): Tell libgc we'll be finalizing
on demand.
(scm_gc): Explicitly run finalizers here. If you're calling this
function, you probably want synchronous GC.
