Follow the XDG spec for configuration

[goranmoomin]

Currently the location of configuration file for mackup is ~/.mackup.cfg, which clutters the $HOME directory. I believe that mackup should closely follow the XDG specification (Summary) and allow users to place configuration files to $XDG_CONFIG_HOME/mackup/config.

[goranmoomin]

This can be made backwards compatible by checking the existence of $XDG_CONFIG_HOME/mackup/config(note that if $XDG_CONFIG_HOME is not set, it fallbacks to ~/.config, so we should also check the existence of ~/.config/mackup/config, and if nonexistent, falling back to ~/.mackup.cfg, while showing a deprecation notice(or not...).

[lra]

This is a good idea.

[treee111]

Ist done, PR's merged and can be closed?

[NorthIsUp]

#1987 to have mackup store its own files in XDG compliance

[con-f-use]

#632 for cross-reference #1318, #473