This generates error pages based on IDs. This allows to reuse errors
and add more advanced information to them.

This patch also makes Icecast send in plain text OR HTML based
on the clients Accept:-string.

This allows ICY source clients to log in again with 2.4.x-style config.
ICY does not provide a username. Therefore the username was unset.
This patch sets the username for ICY source connections and allow them
to be handled as if a username was provided.
Configuration tag <shoutcast-user> was added to select the username to set
to. This defaults to "source".

This fixes make distcheck and the distribution tarballs, those
were previously lacking this header which caused builds to fail.

This will build icecast using Gitlab CI on the Xiph Gitlab with a
Docker image of Alpine Linux.

Changelog and docs still need updating.