Changes I made last week to the task code simplified the task shutdown
and termination model.  Here's how things now work:

When a task is shutdown:

	Any "on shutdown" events for the task are posted

	The "shutting down" attribute of the task is set

	Any attempts to add shutdown events with isc_task_onshutdown()
	will fail, since the task is already shutting down

Task shutdown can be initiated explicity, via a call to isc_task_shutdown(),
or implicitly, when the following conditions occur:

	The "shutting down" attribute of the task is not set

	The task has no references

	The task has an empty event queue


Task termination occurs when:

	The "shutting down" attribute of the task is set

	The task has no references

	The task has an empty event queue
