Error out on unexpected command line argument

[MichaelEischer]

What is the purpose of this change? What does it change?

rest-server doesn't accept arguments. Thus, error out to prevent wrong usage.

Was the change discussed in an issue or in the forum before?

Fixes https://github.com/restic/rest-server/issues/206

Checklist

• [x] I have enabled maintainer edits for this PR
• [ ] I have added tests for all changes in this PR
• [ ] I have added documentation for the changes (in the manual)
• [x] There's a new file in changelog/unreleased/ that describes the changes for our users (template here)
• [x] I have run gofmt on the code in all commits
• [x] All commit messages are formatted in the same style as the other commits in the repo
• [x] I'm done, this Pull Request is ready for review

[MichaelEischer]

I've changed the changelog entry type to "Change" as it is more of a breaking change than a bugfix.