INT 21 - LANtastic v4.1+ - SET AUTO-LOGIN DEFAULTS
        AX = 5FB6h
        ES:DI -> pointer to ASCIZ default user name, immediately followed by
                ASCIZ password
        BL = adapter number to use for default login attempt
            FFh try all valid adapters
            00h-05h try adapter 0-5 explicitly
Return: CF clear if successful
        CF set on error
            AX = error code
Notes:  call with ES:DI -> two nulls to disable auto-login