util, refactor: Use GetPathArg to read "-conf" value

Summary:
Also "includeconf" values been normalized.

This should have been bakported before D16193.

This is a partial backport of core#24675
https://github.com/bitcoin/bitcoin/pull/24675/commits/1276090705060fcc97072481c2383bbaaa556194

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16199