Draft: filter-a plugin to omit some A records in IPv6 environments

I reversed the functionality of the 'filter-aaaa' plugin to omit A records instead of AAAA records.

This is useful especially in some IPv6-only environments. Sometimes, client software ignores system preferences and choses A over AAAA (e.g. 'NodeJS' does or used to do this). On an IPv6-only system, this will result in a failed connection attempt. If no 'happy eyeballs' is used, this will at least defer connection till after timeout, or worse it will make the connection impossible.