(scm_i_frame_single_threaded): Removed.

(scm_i_frame_single_threaded): Removed.
(scm_i_thread): Removed unused signal_asyncs field.
(threads_mark): Do not mark it.
(guilify_self_1): Do not initialize it.  Do initialize
continuation_root field.
(do_thread_exit): Do not remove thread from all_threads list.
(on_thread_exit): Do it here, after leaving guile mode.
(sleep_level): Removed.
(scm_i_thread_put_to_sleep): Leave thread_admin_mutex locked when
returning.  Do not support recursive sleeps.
(scm_i_thread_wake_up): Expect thread_admin_mutex to be locked on
entry.  Do not support recursive sleeps.