Add IP6T_SO_* options definition

[zonyitoo]

Closes #1650

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @gnzlbg (or someone else) soon.

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[alexcrichton]

Thanks for the PR here, but right now there's not many maintainers to help merge this, so it may take awhile.

[JohnTitor]

Thanks!