decoder_load_replay_gain() will create the InputStream anyway, so
we're not saving any overhead by opening the InputStream on demand
only.

Give MPD clients access to the error condition.

Fixes memory leak after InputStream::Check() failure.

Remove that surprising rule.

If the song tag comes from a stream, and MPD playback restarts, MPD
would believe the tag should override the newly received tag.  This
makes the previous tag appear stuck.  This change passes the song tag
only if it's authoritative - i.e. if it's a song file.

Use the new uri_get_suffix() overload that removes the query string.

Make the "open" method of plugins "file" and "archive" dummy methods
that always fail.  Instead, let InputStream::Open() hard-code access
to these two plugins by using OpenLocalInputStream().  This allows
simplifyin the algorithm for falling back to probing archive plugins.

Replaces the method Close().

Don't use the mapper - all DetachedSong instances we get have already
been mapped.

For debugging.

Prepare for UPnP songs that retain there database identity.

From now on, struct Song will be used by the database only, and
DetachedSong will be used by everybody else.  DetachedSong is easier
to use, but Song has lower overhead.

.. instead of decoder_plugin_from_suffix().  This reduces overhead by
walking the array only once.