INT 61 - PC/TCP kernel v2.05+ - "set_option" - SET AN OPTION ON A DESCRIPTOR
        AH = 20h
        BX = network descriptor
        CX = length of buffer (usually 04h)
        DS:DX -> buffer containing option
        SI = (ignored by v2.2-)
        DI = option to set (see below)
Return: CF clear if successful
        CF set on error
            AX = error code (see INT 61"PC/TCP")
Note:   Beame&Whiteside's BWPCTCP shim only supports options 01h and 0Bh; all
          others return CF clear
SeeAlso: INT 61"PC/TCP",INT 61/AH=00h"PC/TCP",INT 61/AH=21h

Values for option to set:
 01h    set non-blocking mode if non-zero
 02h    timeout of call in milliseconds
 03h    user-defined 4-byte magic cookie (not used by kernel)
 04h    TCP window or UDP buffer count (WORD, unsigned)
 06h    do TCP keep-alives if non-zero
 09h    (v2.1+) set IP precedence
 0Ah    (v2.1+) set IP type of service
 0Bh    use a privileged port if port = 0
 0Ch    turn off TCP PUSH bit and don't flush buffer every write (see AH=1Ah)
Note:   use "C" true or false values for boolean options

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