* threads.c (do_thread_exit, scm_cancel_thread,

	* threads.c (do_thread_exit, scm_cancel_thread,
	scm_set_thread_cleanup_x, scm_thread_cleanup): Lock on thread-specific
	admin mutex instead of `thread_admin_mutex'.
	* threads.h (scm_i_thread)[admin_mutex]: New field.
	* throw.c (make_jmpbuf): Don't enter critical section during thread
	spawn -- there is a possibility of deadlock if other threads are
	exiting.