data endpoints. The control endpoint doesn't need read/write/poll
operations, and more importantly, the thread counts should be
separate so that the control endpoint can properly reference itself
while deleting and recreating the data endpoints.
* Add some macros that handle referencing/releasing devices, and use them
for sleeping/woken-up and open/close operations as apppropriate.
* Use d_purge for FreeBSD, and a loop testing the open status for all
the endpoints for NetBSD and OpenBSD, so that when the device is
detached, the right thing always happens.