Reject unsupported flags

[wismill]

Fail whenever an unsupported flag is used. See the discussion in #932

See also #942 to check valid flags properly.

@bluetech @whot this is probably better to fail explicitly (and early) rather than lie to the user.

It also enable indirectly to check for supported flags.

TODO:

• Single changelog entry
• Test all the functions with unsupported flags

[bluetech]

There is a problem that this change is in itself a breaking change. Might be fine though?

[wismill]

I agree, but it seems the only proper behavior (see the description). Currently some functions follow this behavior and other do not. This MR makes the behavior consistent across the code base.

[whot]

There is a problem that this change is in itself a breaking change. Might be fine though?

IMO it's fine - functions not checking their arguments is not something a caller should be relying on.

Code LGTM though I think the various enums are unnecessary (see my inline comments). Thanks for fixing this.

[whot]

Does this work?

found_path:
    {
        const bool ok ...
        flcose
        if (!ok) {
        }
    }

[wismill]

It will work, it’s just that I have some hope to switch to C23 for the basic features: constexpr and attributes would be great appart lifting this label restriction!

[whot]

I'll leave this up to you then, whichever you prefer. all LGTM now, thanks

[wismill]

Rebased & applied review