Instead of UV_UDP_IPV6ONLY we have now UV_UDP_DUALSTACK, which will try to turn
on the dual stack behaviour.
Make UV_UDP_IPV6ONLY the default behaviour on systems supporting dual…
Only if the system doesn't support dual stack and the UV_UDP_DUALSTACK
flag is set, it will through an error.
Add windows support for UV_UDP_DUALSTACK.
What is the motivation for this change? Most platforms are dual stack by default.
Except the platforms which don't support dualstack.
So I have a different behaviour of uv_udp_bind6(handle, addr, 0) on platforms which support it and on platforms which don't support it (on one it will be dualstack, on the other it will be only ipv6).