os: clean up checking for IPv6
- drop some unnecessary / obsolete ifdef's
- explicitly check whether AF_INET6 is available (instead of disabling it silently)
- drop unncessary extra check for AF_INET6 all over the sources (just rely on IPv6 define)
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The migration is almost done, at least the rest should happen in the background. There are still a few technical difference between the old cluster and the new ones, and they are summarized in this issue. Please pay attention to the TL:DR at the end of the comment.