INT 21 - LANtastic - SEND UNSOLICITED MESSAGE
        AX = 5F98h
        DS:SI -> message buffer (see below)
Return: CF clear if successful
        CF set on error
            AX = error code
Note:   v4.1- return no errors

Format of message buffer:
Offset  Size    Description
 00h    BYTE    reserved
 01h    BYTE    message type
                00h general
                01h server warning
                02h-7Fh reserved
                80h-FFh user-defined
 02h 16 BYTEs   ASCIZ destination machine name
 12h 16 BYTEs   ASCIZ server name which user must be logged into
 22h 16 BYTEs   ASCIZ user name
 32h 16 BYTEs   ASCIZ originating machine name (filled in when received)
 42h 80 BYTEs   message text