* libguile/init.h:
* libguile/init.c (scm_i_init_guile): Change arg to this internal
  function from SCM_STACKITEM* to void*.  Actually it's a
  struct GC_stack_base*.

* libguile/bdw-gc.h: Don't do pthread redirects, because we don't want
  to affect applications' pthread_* bindings.

* libguile/pthread-threads.h (scm_i_pthread_create)
  (scm_i_pthread_detach, scm_i_pthread_exit, scm_i_pthread_cancel)
  (scm_i_pthread_sigmask): Do pthread redirects here, in this internal
  header.

* libguile/threads.h: Remove declaration of internal
  scm_i_with_guile_and_parent.  Remove declaration of undefined
  scm_threads_init_first_thread.  Make declaration of internal
  scm_threads_prehistory actually internal, and take a void* (actually a
  struct GC_stack_base*).

* libguile/threads.c (GC_get_stack_base): Implement a shim if this
  function is unavailable, and fold in the implementations of
  get_thread_stack_base.
  (GC_call_with_stack_base): Actually implement.
  (guilify_self_1): Take a GC_stack_base* as an arg.
  (scm_i_init_thread_for_guile): Likewise, and set up libgc for
  registration of other threads.
  (scm_init_guile): Use GC_get_stack_base instead of our own guesswork.
  (with_guile_and_parent, scm_i_with_guile_and_parent): Rework to
  trampoline through a GC_call_with_stack_base.
  (scm_threads_prehistory): Pass the "base" arg on to guilify_self_1.