Load rc from XDG_CONFIG_HOME instead of ~/.config

According to the XDG Base Directory Specification:

$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.

This was also noted by Github user lipidity at
https://github.com/stumpwm/stumpwm/pull/117#issuecomment-52278942