0.8.15.7

0.8.15.7
	Threading fixes :

	- bind *restart-clusters* *handler-clusters*
          *condition-restarts* at thread entry: inter-thread restarts
	  don't work (nor is it clear what they'd do if they did)

	- threads exit when their initial function returns, no need to
          call unix-exit (which may do interesting things with file
          buffers that we'd rather didn't happen)

	arrange_return_to_lisp_function wasn't restoring esp
	properly.  Not sure it ever makes a difference in practice,
	but fix it anyway.