lib/selection: improve IPv6 avoidance if broken

It was still possible to get into a deadlock here.
https://forum.turris.cz/t/not-connecting-to-applications-like-discord/17111/7
If A records for a NS fell out of cache but AAAA remained,
with probability 1-\epsilon we'd choose an AAAA address
even if IPv6 was considered broken.

I looked at *the whole* no6 strategy again, and I do think that
there are no such holes anymore.  A few percent attempts will still
go over IPv6 even if it's considered broken, but that sounds OK-ish.