Keep track of threads waiting in kse_release() to avoid a race

Keep track of threads waiting in kse_release() to avoid a race
condition where kse_wakeup() doesn't yet see them in (interruptible)
sleep queues.  Also add an upcall check to sleepqueue_catch_signals()
suggested by jhb.

This commit should fix recent mysql hangs.

Reviewed by:	jhb, davidxu
Mysql'd by:	Robin P. Blanchard <robin.blanchard at gactr uga edu>