[JENKINS-65911] Correct JNA method signature for fcntl(2)

[basil]

While looking at JENKINS-72833 I remembered my old friend JENKINS-65911. While I could no longer reproduce the problem on my macOS system, I was able to reproduce incorrect fcntl results in a new unit test when running on macOS (but not Linux). The problem seems to be, as I wrote back in 2021, an incorrect JNA signature. fcntl is a variadic function. While I couldn't find much documentation about this, I tried using Java varargs and hoping JNA would deal with this correctly, and it did. My unit test started passing on macOS and there were no regressions running the test on Linux. Just to be sure, I stepped through the restart sequence outside of systemd in a debugger and verified in a separate terminal running strace that the system calls on Linux were unchanged. While I was here I also wrote some other tests to increase coverage for this file.

Testing done

Before this PR, new unit test failed on macOS and passed on Linux. After this PR, passes on both platforms.

Also stepped through the restart sequence outside of systemd on Linux and verified correct system call input and output in strace.

Proposed changelog entries

• Fix a crash when restarting Jenkins on macOS.

Proposed upgrade guidelines

N/A

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@mention

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[basil]

https://github.com/jart/cosmopolitan/blob/1.0/libc/sysv/consts.sh#L170

[NotMyFault]

Interesting finding and analysis 👀

[StefanSpieker]

Thanks for the reasoning and detailed explanation

[MarkEWaite]

This PR is now ready for merge. We will merge it after approximately 24 hours if there is no negative feedback.

[timja]