libusbk backend

[Vitali Lovich]

Ported Pete's libusbk backend work on-top of rc3 (https://github.com/vlovich/libusb/tree/windows_libusbk_support).  This is a straight cherry-pick from my branch (which works) & untested, so I'm not sure if it actually still works (although I think it should).  It probably also requires the windows pollfd fixes (https://github.com/vlovich/libusb/tree/win32-poll) to work properly with a libusb-1.0 app ported to windows.

I'm planning this for inclusion for the first forked release of libusbx.

-Vitali