kill: return 1 and gnu style stderr in case of no pid

[jadijadi]

including a new test for "no pid provided" use case.

closes #6221

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/cp/same-file. tests/cp/same-file is passing on 'main'. Maybe you have to rebase?
Skipping an intermittent issue tests/tail/inotify-dir-recreate (passes in this run but fails in the 'main' branch)
GNU test failed: tests/cp/cross-dev-symlink. tests/cp/cross-dev-symlink is passing on 'main'. Maybe you have to rebase?

[BenWiederhake]

Does exactly what is says on the tin. Thank you!

Can I ask you to change the excessive indentation?

(Also, sorry about the "review self-request", I fully expected it to be an error message, because self-requesting makes to sense to me, and I was curious how github handled that edge case.)

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/cp/same-file. tests/cp/same-file is passing on 'main'. Maybe you have to rebase?
GNU test failed: tests/cp/cross-dev-symlink. tests/cp/cross-dev-symlink is passing on 'main'. Maybe you have to rebase?

[jadijadi]

Does exactly what is says on the tin. Thank you!
Can I ask you to change the excessive indentation?

Thank you @BenWiederhake . Also applied your suggested change.

[cakebaker]

Thanks!

[sylvestre]

@jadijadi thanks for sharing your experience here :) https://www.youtube.com/watch?v=wzWNkNPHJx0