kqueue: Fix missing wakeup

kqueue: Fix missing wakeup

While one thread is running kqueue_scan(), another thread can begin
scanning the same kqueue, observe that the event queue is empty, and
go to sleep. If the first thread re-inserts a knote for re-processing,
the second thread can miss the newly pending event. Wake up the kqueue
after a re-insert to correct this.

This fixes a Go test hang that jsing@ tracked down to kqueue.

Tested in snaps for a week.

OK jsing@ mpi@