mdns4_minimal lets later DNS module handle IPv6

[danielzgtg]

Opening as requested by @pemensik. I'm pursuing systemd/systemd#39800 instead for my use case. The only fix in nss-mdns I can see is, as I said:

The only solution [here] is changing UNAVAIL to NOTFOUND upon detecting .local, but that's an actual breaking change affecting the behavior of /etc/nsswitch.conf for others whose intranet DNS server responds to .local.

[pemensik]

I think it would make sense to emit NOTFOUND for all requests specifically requesting IPv6. I doubt this situation is desired and expected. Something similar happens for example to dnsmasq users. They define null address only in IPv4 and expect it will block all queries to such name. But AAAA queries are forwarded, not handled locally.

I think it would make sense to return NOTFOUND for names in mdns4_minimal and mdns6_minimal, for the address family not handled by the query. If AF_UNSPEC request is processed, wait only for handled family.

[evverx]

I doubt this situation is desired and expected

Given how old this stuff is I think it's likely that there are setups somewhere where that sort of thing is expected and relied on.