run finalizers asynchronously (in asyncs and/or pthreads)

run finalizers asynchronously (in asyncs and/or pthreads)

* 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.