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.

Eliminate the std::string bloat.

Forward the Error to the caller.

Reduce bloat.

Pass "const char *" to Expat.

Reduce runtime bloat.

Reduce overhead by omitting the part of the buffer that consists only
of whitespace.