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.

Not necessary because we control the LightSong's life cycle.

Reduce bloat.  We never used the set lookup.

Reduce overhead.

Don't allocate the object when it's not needed.

The caller should better obtain the object id from the returned
UPnPDirObject instance.

If Namei() succeeds, the object id is guaranteed to be set.

Reduce runtime overhead.

We know already that vpath is empty, let's use this instance.