- Implement autochanger drives protocol so that Dir knows

  how many drives an autochanger has.
- Do not request drive number in label, ... if only one drive.
- Turn off debug code.
- Fix update slots to clear slot number of every slot before
  setting it.  This fixes (I believe) bug #471
- Make unmount unload the autochanger slot.
- Modify open() on mount to be read-only and non-blocking,
  otherwise the mount can block for a long time.
- Make a few error message numbers unique.
- Make a few error messages more correct.
- Apply patch from Thorsten to fix Win98 stat() command.