kernel - close numerous kqueue MP and blocking races

kernel - close numerous kqueue MP and blocking races

* create race

	Close a race where one thread registers a kevent and blocks in
	f_attach allowing another thread to register a change or deletion
	on the same event.

* delete race

	Close a race where one thread deletes a kevent and blocks in
	f_detach allowing another thread to register a change or deletion
	on the same event.

* scan/close race

	Close a race where one thread is in the middle of a kqueue_scan()
	and another concurrently close()s a related file descriptor.  This
	could cause select and poll to loop forever due to not being able
	to delete the 'spurious' kevent.

* register/close race

	Close a race where one thread registers a kevent and blocks
	in f_attach and another thread close()s the related file
	descriptor at the same time.