Protocol and feature improvements, prepare for future extensions

[eduardocasino]

Hi,

More than a pull request, this is more a basis for discussion about the evolution of the SD-drive firmware and protocol, as I think we are the only two people actively maintaining it.

I'm finishing a port of CP/M-65 to the KIM-1 with the SD-Shield, which includes an image manipulation utility. While working on it, I missed some functionality that wasn't present or did not work well in the current firmware. I've added this new functionality with backwards compatibility in mind, so a host can query for the new features and work gracefully with older or newer versions. The list of changes is:

• Manage unknown commands properly, discarding all the data from the host and returning a NAK with an error code.
• Validate the drive number in all relevant operations.
• Various bug fixes.
• Re-implementation of the GET_VERSION function. It is not working in the current 1.X firmware anyway, so I've decided that it would be better to return a MAJOR and MINOR byte instead of a string.
  *As we can't rely on the older PC_GET_VERSION function and also calling unknown commands produces a deadlock in versions 1.X, I've bumped the PROTOCOL_VERSION to 2, so it is possible to use the undocumented PR_VERSION_INFO command to distinguish between older and newer firmware.
  *Implement new commands for managing disk images: PC_FORMAT, PC_COPY, PC_RENAME, PC_ERASE.

I know it is a large patch. I can try to break it up if that eases the discussion.

Cheers,