jtbx discovered that MPD is using the Pipewire stream volume for volume
control, but that when Pipewire/Wireplumber restores the stream volumes
on restart, it wrongly assigns the MPD stream volume to the Shaireport
Sync stream as well.
Work around that bug by making MPD mix itself and not through
Pipewire/Wireplumber.
- Move audio configuration into a service module to make it easily
usable by multiple hosts.
- Allow configuration of AirPlay 2 name.
- Don't set MQTT topic of Shairport-Sync (AirPlay 2) explictly. Defaults
to hostname now.
- Don't set sound.enable anymore, since its not needed.
June