Enable IPV6_V6ONLY flag to increase reliability of IPv4 service

[alexrsagen]

This PR fixes #2342.

yhirose/cpp-httplib@b2203bb explicitly disables the IPV6_V6ONLY flag, making the IPv6 socket bind to both IPv4 and IPv6, leaving the IPv4 socket (or the IPv4 part of the IPv6 dual-stack socket?) unable to bind, as mentioned here.

ZeroTierOne explicitly binds to both IPv4 and IPv6, this is not valid while the IPV6_V6ONLY flag is disabled.

To resolve the issue, this PR makes ZeroTierOne not utilize a dual-stack IPv6 socket by explicitly enabling the IPV6_V6ONLY flag.

[laduke]

Thanks for doing this. We'll have test it on every OS. It might take a while.

[alexrsagen]

Hey @laduke, people are still experiencing issues (see zerotier/DesktopUI#51 (comment)) that could potentially be resolved by my PRs.

I'd recommend you guys look into getting this and zerotier/DesktopUI#61 merged. The changes are very small, but could have a big impact.

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