taskqueue_start_threads(struct taskqueue **, int count, int pri,
const char *name, ...);
This allows the creation of 1 or more threads that will service a single
taskqueue. Also rework the taskqueue_create() API to remove the API change
that was introduced a while back. Creating a taskqueue doesn't rely on
the presence of a process structure, and the proc mechanics are much better
encapsulated in taskqueue_start_threads(). Also clean up the
taskqueue_terminate() and taskqueue_free() functions to safely drain
pending tasks and remove all associated threads.
The TASKQUEUE_DEFINE and TASKQUEUE_DEFINE_THREAD macros have been changed
to use the new API, but drivers compiled against the old definitions will
still work. Thus, recompiling drivers is not a strict requirement.
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