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.

Memory leak.

Bloat.

Must not perform blocking I/O while holding a mutex.

Remove the ContentDirectoryDescriptor right away.  Reduces bloat.

Now that MPD forks early, this workaround is obsolete.

Keep the parent process around until MPD has finished initializing.

This is important for libraries that are allergic to fork(), such as
libupnp.