When isc_task_shutdown() is called, any shutdown events are posted
	and the task goes into shutting down state.  isc_task_onshutdown()
	may no longer be called.

	If the task allows transition to the done state (the default), then
	as soon as the task's event queue is empty the task will enter the
	done state.  Once the done state has been entered, events may no
	longer be posted (regardless of the allowsend state).  If transition
	to the done state is not allowed, then the task will continue to
	exist and be able to receive events.

Moved event support to event.[ch].
The final state of a task is now called "done" not "shutdown".
Created a flags variable instead of having separate booleans.
Added isc_task_allowdone() and isc_task_purgerange().
Minor tracing fixes.