refactor: update unix build tags and add plan9

[kruskall]

What does this PR do?

resolve obscure compile errors when trying to compile on different goos.
Use unix build tags and add plan9.

We can't implement uid/gid logic without breaks as plan9 uses strings while the api requires int.
Use plan9.Dup to redirect stderr.

See https://man2.aiju.de/2/dup

On plan9,

• plan9.Dup(oldfd, newfd) == unix.Dup2(oldfd, newfd)
• plan9.Dup(fd, -1) == unix.Dup(fd)

Why is it important?

• !windows build tags is not true and causes misleading compile errors because it implies the file can be used in any goos other than windows while it's meant to be used for unix
• _other has the same problem

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have added tests that prove my fix is effective or that my feature works

Author's Checklist

• [ ]

Related issues

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: 86c1607

History

• 💔 Build #369 failed b4f6f1b

[VihasMakwana]

LGTM