Fix dual-stack socket usage on Windows

[himikof]

The transport code has a hard dependency on dual-stack sockets (using IPv4-mapped IPv6 addresses). Those sockets only work if IPV6_V6ONLY socket option is unset. Unfortunately, the default value for this option is not always false:

• on Windows, it is explicitly documented as true
• on Linux, it is runtime-configurable (using net.ipv6.bindv6only sysctl, fortunately itself defaulting to false)

Set this option explicitly to ensure the socket will work regardless of the default value.

It could be the underlying reason for #168.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 37.95%. Comparing base (5279161) to head (3c987e9).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #247 +/- ## ========================================== - Coverage 38.03% 37.95% -0.08% ========================================== Files 18 18 Lines 3163 3164 +1 ========================================== - Hits 1203 1201 -2 - Misses 1960 1963 +3 ```

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[felinira]

Thanks a lot :)