- Add the appropriate entry points in the kernel
- Add the function skeleton, for now it only checks if the drive exists
- Add the entry point in NEXTOR.SYS (and step it up to v2.11)

(to do: fill FAT sector number and offset)

- Use a dedicated buffer, instead of TMP_FCB, for intermediate data
- Add an explicit flag for FAT12 drives
- Return .NDOS if the drive is neither FAT12 nor FAT16
- Add a flag for unused clusters
- Always return the real value of the cluster (not FFFFH always when it's
  the last cluster in the file)

A new error code is defined (.ICLUS, B0h), it will be returned
when the cluster number supplied to GETCLUS is invalid;
the "valid cluster number" flag has been removed.

Also, some missing Japanese error messages have been added.

…lusters

- It uses the new GETCLUS function to check the FAT chain of
  disk image files  when createing an .EMU file.
- It now accepts a directory as paramneter for "set", then it
  searches a file with the same name and .EMU extension
  inside the directory.