Get settings info out of headers, out of libutil

[Ericson2314]

Motivation

Want to separate the current configuration script and args infra from the core libraries (libstore and libexpr), where it doesn't belong, and push down into libmain.

Context

Do not review by commits

• contains commits from a different PR that are listed here because that PR was squashed
• not much of useful separation anyway in this commit history

Progress on https://github.com/NixOS/nix/issues/5638

Priorities and Process

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[Secrus]

It looks like something in Python 3.13 is different, cause we have similar errors in #9523

[dimbleby]

cf https://github.com/python-poetry/poetry/pull/9523

Seems like something has changed at python 3.13, but in poetry's own pipeline only macos seems affected.

I'm in no position - and not terribly interested - to reproduce, but if you figure out what is going on then probably that would be a useful contribution.

[Secrus]

Resolved in #9523

[github-actions[bot]]

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