date: invalid format items should be emitted verbatim, and not cause an error

[BenWiederhake]

%s is the field for "number of seconds since UNIX epoch", and %ö is the field for "doesn't make sense, so just emit %ö verbatim".

$ date +%?
%?
$ cargo run -q date +%?
date: invalid format %?
[$? = 1]
$ date +%ö
%ö
$ cargo run -q date +%ö
date: invalid format %ö
[$? = 1]

Found while revising #6142.

This issue is closely related, but not identical to, #6398.