Move audio configuration into a service module

- 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.

modules/services/audio/networking.nix

@@ -0,0 +1,16 @@
+{ config, pkgs, lib, ... }:
+
+with lib;
+
+let
+
+  cfg = config.ccchh.services.audio;
+
+in
+
+{
+  config = mkIf cfg.enable {
+    # Disable IPv6, since Shairport-Sync doesn't work with IPv6. Unclear why.
+    networking.enableIPv6 = false;
+  };
+}