This stores the first data sector of the file that's mounted,
relative to the start of the drive (3 bytes, we assume 4th byte
is always 0).

Also, _GDLI temporarily returns that first sector number.

When mounted files were accessed via file access functions some extra
code to save and restore variables was needed since these functions
are not reentrant. Now that the mounted files are accessed via direct
sector access, this extra code is not needed anymore.

Once mounted files are accessed via direct sector access there's no need
to keep the mounted file open. So:

- Remove the code that opens/closes the file on mount/unmount.
- Store the file name in the unit descriptor, for _GDLI.
- Check the read only mount flag explicitly in CALL_UNIT.
- Remove no longer needed auxiliary code and variables.

_MAPDRV will return an .ICLUS error if the file to mount is not stored
across consecutive clusters (i.e. it's fragmented).

Also reword "Invalid cluster number" error message to
"Invalid cluster number or sequence".

* Add new "Invalid cluster sequence" BASIC error to user manual.

* Mention that a file must be stored across consecutive sectors to be
  mountable in the user manual.

* Mention that MAPDRV requires a file to be stored across consecutive
  sectors to be mountable, in the programmers reference.

* Mention that .ICLUS is returned by MAPDRV if a file to be mounted
  is not stored across consecutive sectors,
  in the programmers reference.