Int 2F Fn D101  - Mdebug Command Driver - Initialize Driver                [G]

   AX = D101h
   CX = command driver counter

Return: DL = FFh if successful
       CX incremented
        else error: all registers unchanged

Note:  this function must end with a  far call to the old INT 2F handler after
     changing the registers
   this function must be reentrant