Respect XDG_CONFIG_HOME

[pebolle]

popt looks for .popt in ~/ but should probably look for a popt file in $HOME/.config instead.

From https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html:

$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.

(Yes, this an almost verbatim copy of rpm-software-management/rpm#2153.)

[pmatilai]

Seems like a reasonable request, yup.

[pebolle]

Thanks.

Would it lessen your workload if I try to cobble together a pull request?

(I'm not too familiar with popt but I can, of course, use rpm's XDG_CONFIG_HOME checking code for inspiration so it might not be too difficult. )