on Linux/Unix, handle user config files under the $XDG_CONFIG_HOME path

On Linux/Unix, the current convention moves "hidden" user config files under
~/.config/ , instead of the old ~/ convention.
More precisely, it is defined by the standardized $XDG_CONFIG_HOME environment variable, see https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html

This would be great if the mariadb client tools (particularly the mysql console) could handle $XDG_CONFIG_HOME/.my.cnf too.