date: Fix format optional argument to capture all following parameters

[cerdelen]

Fixed the format argument to capture all following parameters. Also added a regression test.

Fix: #10910

[ChrisDryden]

Was doing an investigation into why this was added in the first place: in #9661 Turns out I added it

It appears that finding unknown arguments would trigger exit() directly so I added that as an attempt to fix that, but then I ended up making the whole thing return a result. LGTM, just waiting on the CI

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/cut/bounded-memory. tests/cut/bounded-memory is passing on 'main'. Maybe you have to rebase?
GNU test failed: tests/factor/t10. tests/factor/t10 is passing on 'main'. Maybe you have to rebase?
Skipping an intermittent issue tests/tty/tty-eof (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/unexpand/bounded-memory is now passing!
Note: The gnu test tests/env/env-signal-handler was skipped on 'main' but is now failing.

[cerdelen]

This also doesn't fix all problems with arguments parsing, but it fixes this false negative at least.

If i just try for example

❯ gdate --date "@$(date +%s)" q
gdate: the argument ‘q’ lacks a leading '+';
when using an option to specify date(s), any non-option
argument must be a format string beginning with '+'
Try 'gdate --help' for more information.
❯ ./target/debug/date --date "@$(date +%s)" q
date: invalid date 'q'

But this is for another pr i'd argue as this PR already fixes a valid way of using date.