Fixes problems with some UPnP servers.

Avoid appending to an obsolete string object.

Not a fatal error.  Fixes assertion failure.

Reduce overhead.

Reduce overhead.

Reduce bloat.

Expat can call CharacterData() multiple times if the CDATA contains
entity references.  We need to collect all of them in one large
string.

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.

Reduce bloat.

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

The UPnP database plugin can now show relative song URIs for remote
songs.

Prepare for UPnP songs that retain there database identity.

Suppresses gcc warnings.

Fall back to UPnPDirObject::url if there is no override.

Remove unnecessary explicit code.

Reduce bloat.

This has been verified already in the caller.

Allows applying map_to_relative_path() with base_uri.

No caller passes a database song to this function, but if he does,
this check doesn't make sense.

This didn't make sense.