USB: On Windows, block to wait for libusb events

USB: On Windows, block to wait for libusb events

This is another attempt at getting the mess that is libusb event
handling on Windows under control. Until libusb makes its HANDLEs
available for polling, we have no choice but to block while waiting
for libusb events. Since we do not want to force drivers to deal
with multi-threading issues, that means we have to block in the
session main loop.

Fortunately, it turns out that our drivers aren't using multiple
event sources, so it is actually possible to block the main loop
without disrupting too much. This also gets rid of the USB thread
on Windows. Thankfully, libusb does not seem to care that we are
now calling libusb_handle_events_timeout_completed() twice per
iteration: first a blocking call (with timeout) in the callback
wrapper, followed by the non-blocking call in the driver-supplied
callback.