Don't allocate the object when it's not needed.

The caller should better obtain the object id from the returned
UPnPDirObject instance.

If Namei() succeeds, the object id is guaranteed to be set.

Reduce runtime overhead.

Reduce bloat.

We know already that vpath is empty, let's use this instance.

The UPnP database plugin can now show relative song URIs for remote
songs.

Prepare for UPnP songs that retain there database identity.

Suppresses gcc warnings.

Fall back to UPnPDirObject::url if there is no override.

Remove unnecessary explicit code.

Reduce bloat.

This has been verified already in the caller.

Allows applying map_to_relative_path() with base_uri.

No caller passes a database song to this function, but if he does,
this check doesn't make sense.

This didn't make sense.

Works around a build failure on Solaris because annoyingly, Solaris
reserves the name "queue".  This rename was pending anyway.

The Song class is only used for database songs now.  A Song without a
Directory is not possible anymore.

Detach the Song class completely from the public API, only to be used
by SimpleDatabase and the update thread.

Check mpd_response_finish() before using mpd_song.  Don't skip this
check even if the mpd_song is non-nullptr.

Simpler to use than const_cast.

This saves another 3% memory.

Reduces overhead because we need to manage only one memory allocation.
According to valgrind/massif, we save 7%.

The UPnPDirObject is a temporary object, we can move its contents.
This reduces runtime overhead.

Reduce bloat.

Move code to method Start() and add Error& parameters to several
methods.