INT 61 - PC/TCP kernel v2.05+ - "get_option" - GET OPTIONS APPLIED TO NET DESCR
        AH = 21h
        BX = network descriptor
        CX = length of buffer
        DS:DX -> buffer for return values
        SI = 0004h (ignored by v2.2-)
        DI = option (see AH=20h)
Return: CF clear if successful
            DS:DX -> value (usually 32 bits) returned by selected option
        CF set on error
            AX = error code (see INT 61"PC/TCP")
Note:   this function is not supported by Beame&Whiteside's BWPCTCP v3.0a shim
SeeAlso: INT 61"PC/TCP",INT 61/AH=00h"PC/TCP",INT 61/AH=20h

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