INT 15 - MultiDOS Plus - SEND MESSAGE TO ANOTHER TASK
        AH = 04h
        AL = mailbox number (00h-3Fh)
        CX = message length in bytes
        DS:SI -> message
Return: AH = status
            00h successful
            01h out of message memory
            02h invalid mailbox number
Note:   the message is copied into a system buffer; the caller may immediately
          reuse its buffer