HYPPO-DOS: implement missing DOS features and api

[lydon42]

This will start after Batch 3 release.

It will add and finish:

• dos_detach0/1 HYPPO-DOS: refactor d81attach / d81detach calls #628
• dos_writefile, dos_rmfile, dos_fstat Implemented trap_dos_writefile and trap_dos_rmfile #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.
  • HYPPO-PROC: process descriptor needs to handle sd card paths #765 (see ROOT: Suboptimal SD card path handling in HYPPO and FREEZER mega65-freezemenu#80)
• other missing functions?
• and clarify API
• Implement checking reading/writing offsets by hypervisor DOS against partition boundaries #17
• HYPPO does not map IO correctly for OpenDir and ReadDir #654
• HYPPO: Write-enablement of disk images is useless #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.