Int 60 Fn 05  - Mdebug - Get And Set Mdebug Flags                          [G]

   AH = 05h
   DS:SI -> password or a null byte
   BL = new value for the semaphor of MDEBUG
        00h  enable popup of MDEBUG
        else disable popup of MDEBUG

Return: AX = return code
       FFFEh password is invalid
       FFFDh display mode is invalid
      else successful
       BL = old value of the semaphor of MDEBUG
       BH = old value of the INT 08h semaphor
           (this semaphor is always reset after this function)