libaltos: integrate Windows support.

This adds Windows support for discovery and I/O.

The API to the library is mostly unchanged, except that it now exports
product and vendor USB IDs as Win7 doesn't expose the product name
anywhere that we've been able to find, so we'll be updating the
firmware to use unique idProduct values for each product.

Signed-off-by: Keith Packard <keithp@keithp.com>