This allows specifying, for example, the music directory using path
literals without causing the directory to be copied to the Nix store.
Suggested-by: Silvan Mosberger <infinisil@icloud.com>
MPD is using syslog for its logging output, while it could directly
log to systemd's journal, as this daemon is primarily used as a
systemd user service. This change makes MPD log to standard output,
which is captured by systemd.
See https://github.com/NixOS/nixpkgs/pull/57608, which does the same
thing to NixOS's MPD service.
