Add interface to disable automatic finalization

Add interface to disable automatic finalization

* libguile/finalizers.h:
* libguile/finalizers.c (run_finalizers_async_thunk)
  (finalization_thread_proc): Call the new scm_run_finalizers helper.
  (scm_i_finalizer_pre_fork): Only spawn the thread if automatic
  finalization is enabled.
  (scm_set_automatic_finalization_enabled, scm_run_finalizers): New
  functions.
  (scm_init_finalizers, scm_init_finalizer_thread): Only set a finalizer
  notifier if automatic finalization is enabled.

* doc/ref/libguile-smobs.texi (Garbage Collecting Smobs): Add discussion
  of concurrency.

* doc/ref/api-smobs.texi (Smobs): Document new functions.