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:

• [x] Single changelog entry
• [x] 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.

[wismill]

Rebased & applied review