This commit adds support for URBs (USB Request/Response Blocks). These
allow for submitting multi-transfer sized buffers with USBUS handling
the individual usbdev xmits. Multiple URBs can be queued at once for a
single endpoint and USBUS will handle them in the order of submission.
OUT endpoint URBs must always consist of a whole number of full-sized
transfers (N x MaxEndpointSize). They will automatically finish after
the endpoint received a transfer less than the endpoint size.
IN endpoints can be arbitrary-sized and do not have to consist of a
whole number of full-sized transmissions. They support a flag to
indicate that the last transfer in the sequence must be less than a full
sized transfer (USBUS_URB_FLAG_AUTO_ZLP) and this adds a zero length
transfer at the end of the transmissions if the last transfer was equal
to the maximum transfer size.
URBs can be cancelled, but if the URB is already being processed it will
be cancelled after the current transmission within the URB is finished.
If it is still in the queue it will immediately be removed from the
queue.
Replace direct accesses to sched_active_thread and sched_active_pid with
the helper functions thread_getpid() and thread_get_active(). This serves
two purposes:
1. It makes accidental writes to those variable from outside core less likely.
2. Casting off the volatile qualifier is now well contained to those two
functions
