Use XDG_CONFIG_DIRS too

[jluttine]

Currently, sxiv/exec/... files are searched under XDG_CONFIG_HOME directory:
https://github.com/muennich/sxiv/blob/df35cdcd2d81e7232905cb46c8fd3a3d046d9b1b/main.c#L911

I'd like to suggest to use also XDG_CONFIG_DIRS if the file isn't found under XDG_CONFIG_HOME. This would enable, for instance, system-wide or desktop-environment specific defaults for those files.