Drillbit miner updates

Hi Con & Kano,

We have a few updates for the cgminr Drillbit driver. Encompasses a few developments:

- We have some new Avalon-based Drillbit miners coming out. There are some minor tweaks to our protocol to support them (including a new non-hardware-specific config format), but they're largely the same from cgminer's point of view. The driver supports both the old and new protocol versions.

- Default miner settings are now removed from cgminer - if the user doesn't specify anything then it just leaves whatever defaults are set in the firmware.

- New --drillbit-auto parameter allows cgminer to tweak per-ASIC settings up and down automatically based on error rates (documented in ASIC-README.)

- Small patch to usbutils. With a libusb update we found that on Windows reset attempts could lead to an infinite loop when a board got unplugged. More details in 4cbc2676a396d3804426df9fb36544df0e1fc7ff

There are a few merges & cleanup commits in this history as I was pushing regularly. Happy to rebase and resubmit the PR if that's preferable. Can also split into multiple PRs if that helps, or make any other changes necessary.

Thanks very much for both of your hard work on cgminer!

Angus