0.8.6.16

0.8.6.16
	Tidy up a bit

	GET-FOREGROUND now prints 'Resuming thread n' messages on
	thread swith, to make it slightly harder to get lost

	THREAD-REPL-PROMPT-FUN goes away: it was a kludge anyway and
	most of what it did is done in the standard prompt function

	gc_thread_pid no more.  Don't need sys/ptrace.h.  Delete call
	to mach_init() which doesn't exist anyway.  Ignore
	uninteresting signals in parent thread, and lose
	parent_sighandler