Filter out link local address when getting host IPv6

[benjamreis]

The link local addresses can't be used

[lindig]

I think this filter should be factored out into one of the helper modules and not repeated. This could be a (sub) module that has IPv6 related functions. In general I would like to see the use of an abstract address type that internally represents IPv6 and IPv4 rather than strings. That would be a bigger change but we should avoid having ad-hoc string-based solutions proliferating.

[benjamreis]

So if I mutualize and use this lib: https://opam.ocaml.org/packages/ipaddr/

Would that be okay?
We can then extend bit by bit the use of the lib in other cases.

[lindig]

Yes, using such a library to encapsulate the address would be a good step. I don't know how feasible that is, though.

[psafont]

I believe this won't be merged as-is. It's better to open a new one once the ipaddr library is used. Note that the ipaddr version used has bugs with certain ranges of IPv6 addresses. I'm working to include a fixed version.

[benjamreis]

New version using the Ipaddr lib as asked here: #5463