0.8.6.11

0.8.6.11
	Some clean up with unix signals possible now that we denote
	them with numbers instead of keywords

	Juggled the order in target-thread.lisp to make it compile
	without warning

	Threads now signal SIG_THREAD_EXIT in the parent, not
	SIGALRM.   CLONE_PARENT is no longer used, so the creating
	Lisp thread gets this signal instead of the original C process

	thread-exit_handler is the SIG_THREAD_EXIT handler.  It calls
	the new static function HANDLE-THREAD-EXIT to manipulate
	*SESSION*

	SB!THREAD::*FOREGROUND-THREAD-STACK* ius dead, remove from
	static variables list