user mode workqueue APIs

[andrewboie]

k_work_q APIs have the worker thread run in supervisor mode. k_queues are used to buffer work requests to the queue.

We need to support work queues where the worker runs in user mode. We will need to use a different kind of buffering object which is user mode compatible.

Other kernel object APIs which support offloading to k_work_q will need additional APIs for working with these kinds of queues.