Fix spurious `throw from within critical section' errors

Fix spurious `throw from within critical section' errors

The crux of this problem was that the thread doing a throw, and so
checking scm_i_critical_section_level, was different from the thread
that was in a critical section.

* libguile/async.h (scm_i_critical_section_level): Removed, replaced
  by per-thread critical_section_level.
  (SCM_CRITICAL_SECTION_START, SCM_CRITICAL_SECTION_END): Use
  per-thread critical_section_level.

* libguile/continuations.c (scm_dynthrow): Check per-thread
  critical_section_level.

* libguile/threads.c (guilify_self_1): Init per-thread
  critical_section_level.
  (scm_i_critical_section_level): Removed.

* libguile/threads.h (scm_i_thread): New critical_section_level field.

* libguile/throw.c (scm_ithrow): Check per-thread critical_section_level.