Linux use XDG specs

[wcasanova]

db: /home/$USER/.simplex/simplex_v1_chat.db

https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html

XDG_CONFIG_HOME
    Where user-specific configurations should be written (analogous to /etc).
    Should default to $HOME/.config.

XDG_CACHE_HOME
    Where user-specific non-essential (cached) data should be written (analogous to /var/cache).
    Should default to $HOME/.cache.

XDG_DATA_HOME
    Where user-specific data files should be written (analogous to /usr/share).
    Should default to $HOME/.local/share

I see that it is already #414