libusb: use eventfd

[bapt]

Summary

Simplify libusb code by replace pipe(2) inter thread event mecanism
with eventfd(2).

This simplifies the code.

[bapt]

note that it makes android's adb tool working reliably

[bapt]

it does not fix but get stalled less often

[kevans]

It's a good cleanup, even if it doesn't solve the problem.

lib/libusb/libusb10_io.c

190

I wonder if we should be gating this else behind fds[i].revents != 0. It makes sense not to continue at the beginning unconditionally because we need to drop the reference if we had a device, but my concern in this branch is that we could have raced after poll(2) returned and accidentally drained something we shouldn't have.

The question I'm contemplating in particular is: if we were to avoid it here, would we observe a different state above poll(2) when we re-enter this function? e.g., a device appearing. In the worst case scenario, ignoring anything in the pipe here that wasn't at the time of the poll(2) causes a spurious wakeup and we subsequently drain it.