nixos/avahi-daemon: resolve mdns only over enabled protocols, disable ipv6 by default

see https://github.com/lathiat/nss-mdns#:~:text=in%20such%20a%20situation%20causes%20long%20timeouts%20when%20resolving%20hosts
especially:
> libnss_mdns.so.2 resolves both IPv6 and IPv4 addresses, libnss_mdns4.so.2 only IPv4 addresses and
> libnss_mdns6.so.2 only IPv6 addresses. Due to the fact that most mDNS responders only register local IPv4
> addresses via mDNS, most people will want to use libnss_mdns4.so.2 exclusively. Using libnss_mdns.so.2
> or libnss_mdns6.so.2 in such a situation causes long timeouts when resolving hosts since most modern
> Unix/Linux applications check for IPv6 addresses first, followed by a lookup for IPv4.

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