Small costly mistake.

Remove a lot of boilerplate by using the errgroup package instead of
our own custom cumbersome solutions. Control server lifetime with an
outer Context.

It is best to just make the DNS server listen on all non-loopback IP
addresses from all interfaces. The loopback exclusion is just to
integrate nicely with an eventual DNS cache running on the same host.

v2.0

See merge request !1

Causes a slight reduction in boilerplate, and gets rid of a bunch of
custom logic for command-line parsing that is no longer needed.

It will listen to a stream and export metrics on connection quality
and audio to Prometheus.

And various other minor fixes.

Handle the case when IcecastMount is nil in the template by using the
protobuf getter.