thread_admin_mutex in order not to risk being joined.
(scm_i_thread_put_to_sleep, scm_i_thread_wake_up): Keep
thread_admin_mutex locked during GC.
* pthread-threads.c, pthread-threads.h: Improvements to debugging
functions.
(thread_print): Print thread number as well as address of thread
structure.
(scm_join_thread): Bugfix.
(scm_lock_mutex, scm_try_mutex, scm_unlock_mutex,
scm_timed_wait_condition_variable, scm_signal_condition_variable,
scm_broadcast_condition_variable): Use the low-level API.
(scm_all_threads): Return copy of thread list (to prevent
unintended destruction).
(scm_threads_prehistory): Initialize heap_mutex of fake thread.
