device filtering support

[Pete Batard]

Oh, and just to clarify, the way I see it is that the libusbx background 
process would be instantiated on first libusbx_init() from any app, and 
destroyed on the libusb_exit() from the last app.

I'd probably also add a regular ping (every few seconds or so) to find 
out if there's still at least one libusbx app around (crash, etc.), and 
kill the process if there isn't.

The goal here is to duplicate something that all OSes should provide 
without us having to build it (a list of all USB devices, with all the 
properties we need already cached), but that some OSes (Windows) don't, 
so we duplicate part of what we want from the OS in userspace.

Regards,

/Pete