Add the following functions to abstract away the creation of task threads

Add the following functions to abstract away the creation of task threads
for usb.  I hope that this will eventually be used for generic devices
that need full fledged blocking threads for event processing.

Create a taskqueue:
void usb_ether_task_init(device_t, int, struct usb_taskqueue *);

Enqueue a task:
void usb_ether_task_enqueue(struct usb_taskqueue *, struct task *);

Wait for all tasks queued to complete:
void usb_ether_task_drain(struct usb_taskqueue *, struct task *);

Destroy the taskqueue:
void usb_ether_task_destroy(struct usb_taskqueue *);