This fixes an old bug which caused the "unused" warnings to be
unreliable; only the first block in the list was marked as being
"used", no matter if it was really used, and the rest was never marked
as "used", suppressing all warnings for them.

Modeled after std::filesystem::operator/() from C++17.

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.