Remove AuthDirPublishIPv6 and let AuthDirHasIPv6Connectivity fill its function.

See #4771 for rationale.

Note that this patch does not take suggested changes in #4470 into
account and keeps treating AuthDirHasIPv6Connectivity as an
AUTOBOOL. Thus, bug fixes for that are included here as well.

This is a fix on master, unreleased as of now.