Update scrypt UDP proxy code for Windows compatibility
Upgrade Gridseed driver to support direct serial I/O without libusb.

This commit adds preliminary support for the same dual-interface capability
of the Zeus driver to the Gridseed driver. This is mainly of interest on
Windows systems, where libusb's required alternative drivers can present
undue complexity.

(Note: While direct serial I/O is functional with this commit, its
use on Windows systems will result in poor performance in its current
state. Further commits will address this.)