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Now that MPD forks early, this workaround is obsolete.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.

Prepare for more advanced daemonization code, which will not work with
daemon().  Let's just require fork().  Everybody who supports daemon()
also supports fork().

Kludge to work around DSD playback failure because the
ReplayGainFilterPlugin is always in the filter chain.

Make the DetachedSong(Song) conversion constructor private.  Everybody
should use map_song_detach() which will take over more
responsibilities soon.

Eliminate global variables.

Prepare to refactor some code.

Delete the object when closing the database.