More renaming and cleanup.

Moved the xdg namespace inside the util namespace, for consistency with
util::log and util::fs. Replaced one occurrence of g_get_system_data_dirs with
util::xdg::basedir::DataDirs.
Mark overrides as such.
Fix an error.

This was introduced in bfad2f75e808fa2d1c8bb4bf964029cc5e4b0745.