NAME
       FreeUnit - deallocate the disk

   SYNOPSIS
       FreeUnit( unitNum ), DRResource
                  D0         A6

        void FreeUnit(LONG);

   FUNCTION
        This routine deallocates one of the units of the disk.  It should
        be called when done with the disk.  Do not call it if you did
        no successfully allocate the disk (there is no protection -- you
        will probably crash the disk system).

   INPUTS
        unitNum -- a legal unit number (zero through three)

   RESULTS

   EXCEPTIONS

   SEE ALSO
        FreeUnit()

   BUGS
        Doesn't check if you own the unit, or even if anyone owns it.