HYPPO-DOS: implement missing DOS features and api

[lydon42]

This will start after Batch 3 release.

It will add and finish:

• [x] dos_detach0/1 #628
• [x] dos_writefile, dos_rmfile, dos_fstat #748
• [ ] add subdirectory management
  • [ ] dos_getcwd (the cwd must be something available to other calls, like attach!)
    • [ ] also allow longer filenames? currently 31 is the max.
  • [ ] #765 (see https://github.com/MEGA65/mega65-freezemenu/issues/80)
• [ ] other missing functions?
• [ ] and clarify API
• [ ] #17
• [ ] #654
• [ ] #494

One decision to make will be if we keep dos_detach (detach all images, don't mount disk drives) or if we replace it by dos_detach0 and dos_detach1 calls, or even condense the attach0/1 and detach0/1 into one call each which takes the drive as a parameter.