Block device support

[mach-kernel]

When moving stuff to my IIgs, I would always dd the image off of a CF card, make an edit using CADIUS, and then dd it back. I would like to be able to use block devices directly to avoid doing this.

Changes:

• struct prodos_image has an fd field
  • GetBlockData SetBlockData use it if it is set
• Image format by reading image headers (we scan for 2IMG as the first four bytes for a .2mg image)
• Clean up AppleSingle endian-swap stuff / move to ntohl/s

Usage: For each command that takes <[2mg|hdv|po]_image_path>, you may substitute with a path to a block device.

[mach-kernel]

Someone mentioned being able to use multiple ProDOS partitions (in a manner similar to CiderPress), so that is next up on the list of things to do for this PR.