Avoid polluting config file when "save" (#25395)

Avoid polluting config file when "save" (#25395)

That's a longstanding INI package problem: the "MustXxx" calls change
the option values, and the following "Save" will save a lot of garbage
options into the user's config file.

Ideally we should refactor the INI package to a clear solution, but it's
a huge work.

A clear workaround is what this PR does: when "Save", load a clear INI
instance and save it.

Partially fix #25377, the "install" page needs more fine tunes.