Reduce bloat.

Send "idle" to the other MPD whenever there's nothing else to do and
forward incoming "idle database" events to all our MPD clients.

Allow database plugins to announce that they have been modified.

Ask the DatabasePlugin for stats when the first client requests them,
not at startup.

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.

I wrongfully assumed that each array element is a name and a value
concatenated.

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]

Kludge for libavahi-client quirk.

Add the debug-only flag "virgin" which gets checked by assert()
calls.  Fixes assertion failures when using zeroconf/avahi.

Using a variable-size struct with embedded string is not worth the
trouble here.  There are not so many Directory objects.