INT 21 - LANtastic v4.0+ - SCHEDULE SERVER SHUTDOWN
        AX = 5FC8h
        ES:DI -> ASCIZ server name in form "\\machine"
        DS:SI -> ASCIZ reason string (80 characters)
        CX = number of minutes until shutdown (0 = immediate)
        DX = option flags (see below)
            bit 0: auto reboot
                1: do not notify users
                2: halt after shutdown
                3: shutdown due to power fail (used by UPS)
            bits 4-7: reserved
            bits 8-14: user definable
            bit 15: reserved
Return: CF clear if successful
        CF set on error
            AX = error code