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

Fixes -Winconsistent-missing-override (clang 3.6).

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

This call was unnecessary.  It is a bad idea anyway to block the MPD
process for some arbitrary amount of time.

Eliminate class LibUPnP and move the code to ClientInit.cxx.  Its
initialization function can be called multiple times, but
UpnpRegisterClient() is called at most once.

Add missing server name prefix.

Don't use std::string, reduce bloat.