Add /usr/local/etc/xdg to the default XDG_CONFIG_DIRS

I can see that this directory is non-empty on at least FreeBSD, so we should add it in the same way as we add /usr/local/share to XDG_DATA_DIRS.

While the spec says If $XDG_CONFIG_DIRS is either not set or empty, a value equal to /etc/xdg should be used that seems inconsistent with adding /usr/local/share to the XDG_DATA_DIRS default.

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