INT 6A - DECnet DOS LOCAL AREA TRANSPORT - OPEN SESSION
        AH = D0h
        DH = FFh
        AL = FFh no password
           = 0Fh password at ES:DI
        ES:BX -> LAT session control block (see below)
        ES:DI -> 16-byte blank-padded password
Return: AH = 00h success
            DL = handle

Format of LAT Session Control Block:
Offset  Size    Description
 00h 18 BYTEs   service name
 12h 18 BYTEs   node name (future use)
 24h 18 BYTEs   port name (future use)
 36h    DWORD   -> session stopped post routine
 3Ah    DWORD   -> service table overflow post routine
 3Eh    DWORD   -> transmit post routine
 42h    DWORD   -> receive post routine
 46h    WORD    session status
                04h circuit failure
                08h stop slot received
 48h    WORD    slot state (LAT driver use)
 4Ah    WORD    local credits (LAT driver use)
 4Ch    DWORD   -> VCB (LAT driver use)
 50h    WORD    backward slot (LAT driver use)
 52h    WORD    forward slot (LAT driver use)
 54h    WORD    remote slot ID (LAT driver use)
 56h    WORD    local slot ID (LAT driver use)
 58h    WORD    slot byte count (LAT driver use)
 5Ah    BYTE    remote credits (LAT driver use)
 5Bh 255 BYTEs  transmitted data slot
15Ah    BYTE    number of receive data slots (4 recommended)
15Bh    BYTE    number of occupied slots
15Ch    BYTE    index of next receive slot to use
15Dh    BYTE    index of current receive slot
15Eh    WORD    pointer to first received character
160h  N WORDs   pointers to receive slots (buffers); each is 259 bytes
Note:   set post routines to 0000h:0000h if polled operation will be used