Switch mdns resolver

[silkeh]

Summary

This PR switches to using systemd-resolved as the default resolver, replacing avahi-daemon.

Resolves #2443

Test Plan

Basic resolution:

1. Install packages from this PR
2. Reboot or stop/start the correct services:

   sudo systemctl stop avahi-daemon.{socket,service}
   sudo systemctl restart systemd-resolved
   killall kdeconnectd
   

3. Resolve local hosts in the network:

   resolvectl query --cache=no -p mdns <host>.local
   

4. Resolve device info from a different machine:

   $ resolvectl query --cache=no -p mdns --type=TXT <host>._device-info._tcp.local
   <host>._device-info._tcp.local IN TXT "model=RackMac"
   

KDE connect:

1. Ensure KDE connect is started:

   kdeconnect-cli -l
   

2. Discover from a different device:

   kdeconnect-cli -l
   

3. Do manual resolution on a different device:

   $ resolvectl query --cache=no -p mdns --type=TXT <host>._kdeconnect._udp.local
   <host>._kdeconnect._udp.local IN TXT "type=desktop"
   

Resolution timeouts using time getent hosts -s $service $host against IPv4-only and IPv6-only hosts:

Service	Host	Time
mdns_minimal	IPv4	1.140 s
mdns_minimal	IPv6	0.104 s
resolve	IPv4	1.510 s
resolve	IPv6	0.002 s

Note that the increased delay seems mostly because systemd also uses jitter (up to 250 ms per request). This results in mDNS timeouts theoretically taking between 875 ms and 1625 ms. Realistically it's more around 1.5 s.

Checklist

• Package was built and tested against unstable

[silkeh]

I had a couple of issues that cropped up when switching:

• systemd-resolved has a 'Hostname conflict' caused by:
  • Receiving its own record on a different VLAN via the mDNS reflector running on my router. Fixed by configuring one interface with connection.mdns=yes and the rest with connection.mdns=resolve via nmcli. (edit: this might have been caused by KDE connect, I can't reproduce it anymore).
  • KDE connect. This service was already conflicting with Avahi (see this upstream issue). I've added a commit to disable the built-in mDNS announcer in KDE connect until it can play nice.
• Kerberos slowed down because of TXT _kerberos.local queries slowing down. Resolved by setting dns_lookup_realm=false (which should probably have been the case already).

[ermo]

The avahi, baselayout and samba changes look sane to me.

[silkeh]

I've dropped the baselayout change as that can't be updated, and it actually works with the current configuration.

[silkeh]

This is currently blocked on compatibility:

• The Avahi D-Bus API. See Option to mirror Avahi D-Bus API systemd/systemd#33298. Another solution might be create a custom translator for this, like this blog post describes (unfortunately the last post in the series).
• The Avahi libraries. See Avahi compatibility libraries systemd/systemd#21669.