feat: allow logging to stdout, stderr

[HeikoSchlittermann]

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

The --log option accepts "stdout", "stderr" as filenames. This prevents rest-server from opening these files, it simply writes the logs to the STDOUT or STDERR stream provided by the caller.

BREAKING in case the user really used "stdout", "stderr" to specify file names, you'll need to update your rest-server invocation to use "./stdout", "./stderr".

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

No.

Checklist

• [x] I have enabled maintainer edits for this PR
• [ ] I have added tests for all changes in this PR
• [x] I have added documentation for the changes (in the manual) Not in the manual, but in the options help
• [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]

What is the use case for logging to stderr? For stdout a pretty common name would be "-", that is --log -.

[HeikoSchlittermann]

I currently do not have a use case for logging to STDERR. I suggested it for both (stderr, stdout) just for completeness. Logging to STDOUT though I have a use case for: I'd like to have Systemd capture all the output. Depending on the invocation, rest-server can't (re)open STDOUT (permissions).

I'm fine with --log - too. But IMHO --log stdout is more self-explanatory. (Using --log /dev/stdout with built-in magic would assume, that /dev/stdout is identical to the STDOUT provided by the caller.)