fix Windows DF option

[jesseklm]

using IP_DONTFRAGMENT from https://github.com/MicrosoftDocs/win32/blob/docs/desktop-src/WinSock/ipproto-ip-socket-options.md

Indicates that data should not be fragmented regardless of the local MTU. Valid only for message oriented protocols. Microsoft TCP/IP providers respect this option for UDP and ICMP.

with value from here https://github.com/tpn/winsdk-10/blob/master/Include/10.0.14393.0/shared/ws2ipdef.h

#define IP_DONTFRAGMENT 14 // Don't fragment IP datagrams.