Int 61 Fn 20  - PC/tcp Kernel V2.05+ - "set_option" - Set An Option On A   [N]

   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

See Also: 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