INT 15 - MultiDOS Plus - EXECUTE A MULTIDOS PLUS COMMAND
        AH = 0Fh
        DS:BX -> ASCIZ command
Return: after command has been processed
Notes:  specified string is executed as if it had been typed at the MultiDOS
          command prompt
        the task is placed on a queue which MultiDOS examines periodically and
          is suspended until MultiDOS has processed the command
        all lowercase characters up to the first blank are converted to upper
          case within the given buffer