-Wformat-nonliteral

This is a follow-up to CVE-2026-16240 (commit 4a18e9b5f36).

That issue was found by -Wformat-nonliteral. That warning option flags
a bunch of other places, which were fortunately not security issues. I
think this is quite a large surface area to leave unchecked. I propose
to fix these and turn this warning option on by default. While this
does require introducing a bit more duplication, the patch (see patch
0003) overall does not blow up the code much at all (911 insertions(+),
896 deletions(-)).

This also runs into some workarounds we had installed for strftime by
explicitly *not* using literal format strings. The y2k-related one of
these is definitely obsolete and can be removed (see patch 0001). The
MinGW-related one (see patch 0002) is also obsolescent if we want to
assume gcc 14+ for MinGW. I think for MinGW we tend to opt for only
supporting recent toolchains, so this might be ok. Otherwise, we could
work around this with some pragmas.