Fix startup failure on Windows 8.1 due to unsupported IPV6_USER_MTU socket option...

Fix startup failure on Windows 8.1 due to unsupported IPV6_USER_MTU socket option being set

Newer mingw-w64 (starting from 8.0.1) introduces support for `IPV6_USER_MTU` socket
option [1], which is not supported on Windows 8.1 and older [2]. As there is no way
to avoid this socket option from being picked at compile time when targeting older
versions of Windows, check for `setsockopt(..., IPV6_USER_MTU, ...)` failures at
runtime in order to avoid startup failure on those versions of Windows where the
`IPV6_USER_MTU` socket option is unsupported.

[1]: mirror/mingw-w64@e30bff4
[2]: `WSAGetLastError()` returns `WSAENOPROTOOPT` (`Bad protocol option`) error code