Improvements to support for BitBurner boards

I've made some improvements to the BitBurner support in the Avalon driver:

The main change is that I've implemented --bitburner-fury-options to allow avalon-options to be overridden for
BitBurner Fury Boards running the new firmware.  (For backwards compatibility the Fury boards will still resepect --avalon-options if the new option isn't supplied.)  Without this it's pretty much impossible to mine with Avalon-based and Bitfury-based Bitburner boards on the same cgminer instance.  (And using separate instances isn't very convenient, either, since they use the same driver.)

I also implemented more sensible defaults for BitBurner boards, so cgminer should do something
sensible if invoked without any command line args (at least if they're running new firmware).

I have no connection with the Bitburner project - I just implemented this because I needed it myself.  I was going to hold off submitting the pull request until I'd been in touch with someone42, but since it appears other people are waiting for this I might as well submit it without delay.

roy