Make kqpoll_dequeue() usable with lazy removal of knotes

Make kqpoll_dequeue() usable with lazy removal of knotes

Adjust kqpoll_dequeue() so that it will clear only badfd knotes when
called from kqpoll_init(). This is needed by kqpoll's lazy removal
of knotes. Eager removal in kqpoll_dequeue() would defeat kqpoll's
attempt to reuse previously established knotes under workloads where
knote activation tends to occur already before next kqpoll scan.

Prompted by mpi@