Support modern locations for configuration files for Unix (XDG_CONFIG_HOME and ~/.config/)

[Ma11ock]

Modern Linux/BSD systems have moved their configuration files out of the home directory and into ~/.config, or the directory pointed to by the environment variable XDG_CONFIG_HOME. This patch adds support for these directories. Tested on Linux and Windows.

[bones-was-here]

I think XDG support should include putting the dlcache in ~/.cache because otherwise it will end up in ~/.config which means that forevermore DP will have to look for it there (even after support for ~/.cache is added).

Please add a --signoff to your commit(s) (to clarify you are releasing the code as GPLv2+).

@Cloudwalk9 is there anything else that needs to be changed at the same time?

[bones-was-here]

Marking as Draft as it can't be merged in the current state.
All the issues mentioned by div https://gitlab.com/xonotic/darkplaces/-/merge_requests/10 and I https://gitlab.com/xonotic/darkplaces/-/issues/16 will need to be addressed.