wish: support for XDG_HOME_STANDARD-- put config files under ~/.config

[markstos]

Thanks for Teamocil.

According to the Free Dektop XDG Base Directory Specification, ~/.config/ should be used for personal config files. From the spec:

$XDG_CONFIG_HOME defines the base directory relative to which user specific configuration files should be stored. If $XDG_CONFIG_HOME is either not set or empty, a default equal to $HOME/.config should be used

systemd also supports the standard, and documents in man file-hierarchy under HOME DIRECTORY:

~/.config
    Application configuration and state... If an application finds $XDG_CONFIG_HOME set, it should use the directory specified in it instead of this directory.

As a user, I appreciate having configuration file standardized in the .config directory, with less litter in my top level directory. For example, I can easily specify that all files under ~/.config should be included in a backup, while all files under /.cache (also standardized) don't need to be backed up.

The current location can still be checked as a backup for backwards compatibility.