Add XDG Base Directory support

Magnus Andersonrequested to merge
magnustesshu/psensor-xdg-base-directory:xdg-base-directory into master

This code will check if the legacy directory exists; if not, it will create either $XDG_CONFIG_DIRECTORY/psensor/ or $HOME/.config/psensor/, and use that. If neither of these directories work, it will also fall back to $HOME/.psensor so there is no chance that this breaks anything.

In addition, lib/slog obeys whatever get_psensor_user_dir() returns, rather than having its own implementation for how to find the correct log file.

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