INT 2F - network - SET NETWORK EVENT POST HANDLER
        AX = B804h
        ES:BX -> new event post handler
Notes:  used in conjunction with AX=B803h to hook into the network event post
          routine
        The specified handler is called on any network event.  Two events are
          defined: message received and critical network error.
SeeAlso: AX=B904h

Values post routine is called with:
        AX = 0000h single block message
            DS:SI -> ASCIZ originator name
            DS:DI -> ASCIZ destination name
            ES:BX -> text header (see below)
        AX = 0001h start multiple message block
            CX = block group ID
            DS:SI -> ASCIZ originator name
            DS:DI -> ASCIZ destination name
        AX = 0002h multiple block text
            CX = block group ID
            ES:BX -> text header (see below)
        AX = 0003h end multiple block message
            CX = block group ID
        AX = 0004h message aborted due to error
            CX = block group ID
        AX = 0101h server received badly formatted network request
            Return: AX = FFFFh (PC LAN will process error)
        AX = 0102h unexpected network error
            ES:BX -> NCB (see INT 5C)
        AX = 0103h server received INT 24 error
            other registers as for INT 24, except AH is in BH
            Return: as below, but only 0000h and FFFFh allowed
Return: AX = response code
            0000h user post routine processed message
            0001h PC LAN will process message, but message window not displayed
            FFFFh PC LAN will process message

Format of text header:
Offset  Size    Description
 00h    WORD    length of text (maximum 512 bytes)
 02h  N BYTEs   text of message
Note:   all CRLF sequences in message text are replaced by 14h