Fully respect xdg base dir spec

[mijoharas]

Hopefully this isn't too controversial, but the xdg base dir specification states that:

If $XDG_CONFIG_HOME is either not set or empty, a default equal to $HOME/.config should be used.

ref from specification (similar for cache dir.).

We continue to check whether the old config directory is present first so that if people upgrade, they won't need to change anything, but otherwise, we default to following the specification.

[mijoharas]

(Note: feel free to squash the commits, I started by creating a new env var, but figured since that required users wanting to stick with the old version to set some env vars, and would thus be a bit of a change for existing users I changed to what's there now. I left the commits as is)