A shutdown event specification is no longer part of the
	isc_task_create() API.

	Multiple shutdown events can be requested with isc_task_onshutdown().
	Shutdown events are posted LIFO.

	The ability to send events can be enabled and disabled with
	isc_task_allowsend().

	Event actions return void; the only way to shutdown a task is to
	call isc_task_shutdown().

	There are no implicit event discards anymore.  (There is no need for
	them, since event actions cannot request immediate shutdown anymore.)