We have removed all libupnp logging calls, and we don't need to debug
libupnp.

Look up attributes in the "atts" array.  Reduce bloat.

Reduce bloat.

Reduce bloat.

Reduces bloat.

We only need it inside StartElement().  Reduces bloat.

No need to handle this in EndElement() - in StartElement(), we already
have all we need.

Reduce excessive bloat.

At the same time, rename the enum types and the class attributes, and
add an "UNKNOWN" type/class.  The latter avoids the "-1" hack.

Return the return value, instead returning it in a reference
parameter.  Reduces bloat by reducing unnecessary std::string usage.

Not necessary or useful.

Reduces bloat by eliminating one std::map.

[mk: renamed source files, applied coding style, reduced bloat, using
MPD's threading library, using MPD's error reporting and logging
library and refactoring, fixed lots of bugs]

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.

Refactor SimpleDatabase::GetLastModified() to be generic for all
plugins.  Remove the SimpleDatabase assumption from db_stats_print(),
allowing it to be implemented by all database plugins.