Allows database plugins to use the EventThread, e.g. for CURL
integration.

The old struct config_param remains only for top-level string options.

Move code from CreateConfiguredDatabase() and add XDG support.  This
implements an automatic Linux fallback for the setting "db_file" if
none was specified.

Add class Context which wraps the Android/Java Context class and add a
JNI wrapper for method Context.getCacheDir().

The new method IsPlugin() replaces the "is_simple" flag.

This is misconfiguration, and MPD should abort.