add atfork interface

* libguile/posix.h:
* libguile/posix.c (scm_c_atfork): New interface.
  (scm_fork): Wrap fork calls in atfork pre/post invocations, and
  finally a GC_call_with_alloc_lock.  It's an attempt to grab the
  interesting mutexes in Guile.

* libguile/weak-set.c (make_weak_set):
* libguile/weak-table.c (make_weak_table): Use atfork mechanism to lock
  and unlock weak sets and weak tables during a fork(), in such a way
  that does not prevent those tables from being gc'd.

libguile/init.c

@@ -383,6 +383,7 @@ scm_i_init_guile (void *base)
 
   scm_storage_prehistory ();
   scm_threads_prehistory (base);  /* requires storage_prehistory */
+  scm_weak_set_prehistory ();        /* requires storage_prehistory */
   scm_weak_table_prehistory ();        /* requires storage_prehistory */
 #ifdef GUILE_DEBUG_MALLOC
   scm_debug_malloc_prehistory ();