0.8.6.10

0.8.6.10
	The diff is kind of noisy, but it's really pretty simple

	The goal here is to deal with multiple 'session's in a
	slightly more sendible way, and for QUIT to kill off
	background threads if any are running

	We create a SESSION struct with various locks and queues,
	so replacing about 4 special variables with one *SESSION*
	structure object.

	MAKE-THREAD adds the new thread id to the current session

	WITH-NEW-SESSION removes the current thread from the session,
	then creates a new session containing only the current thread
	and rebinds *SESSION* to it.  It's used by MAKE-LISTENER-THREAD
	and is also exported albeit in a "we reserve the right to
	change this but are unlikely to" fashion

	TERMINATE-SESSION terminates all threads in the session other
	than the caller.  %END-OF-THE-WORLD-HANDLER calls it

	Also exported *INVOKE-DEBUGGER-HOOK* from SB-EXT for SLIME and
	similar alternate debugging interfaces to use.