INT 2F - MDEBUG command driver - EXECUTE KEY IN THE MONITOR
        AX = D103h
        BX = key code (like result of an interrupt 16h call)
        CX = 0 -> the cursor is in the ASCII column of the monitor
        CX = 1 -> the cursor is in one of the hex fields of the monitor
        DS:SI -> MDEBUG data structure (see AX=D102h)
        ES:DI -> actual byte in the monitor
Return: AL = FFh
        AH = return code
            00h key processed, read next key
            01h leave MDEBUG popup session
            02h leave MDEBUG popup session and automatically popup again if DOS
                is not busy
            03h signal an error (beep)
            04h driver has redefined the key, proceed with the new key
                BX = new key code
                MDEBUG will not pass the new key to the command driver(s)
           else treat like code 00h
Note:   this function must end with a far call to the old INT 2F handler (with
          registers unchanged) if the driver does not support the key in BX.
          Otherwise, the driver must not chain to the old INT 2F.