kill: print signals vertically when using --list flag

[m-haisham]

Environment: Ubuntu 20.04, uutils main branch (git commit 415de28), GNU coreutils version 9.5.3-8f39 (from git commit coreutils/coreutils@8f3989d).

See #6197 for more details.

[BenWiederhake]

Hah! Short and sweet :D

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/misc/kill. tests/misc/kill is passing on 'main'. Maybe you have to rebase?
Skip an intermittent issue tests/rm/rm1 (fails in this run but passes in the 'main' branch)
Congrats! The gnu test tests/misc/kill is no longer ERROR!
GNU test failed: tests/cp/cross-dev-symlink. tests/cp/cross-dev-symlink is passing on 'main'. Maybe you have to rebase?

[m-haisham]

Accidentally pushed table formatting code to this branch, I've rolled it back.

[BenWiederhake]

Looks good.

[m-haisham]

Failed check does not seem to be related to this tool. @BenWiederhake

[BenWiederhake]

Thank you!

[sylvestre]

it would have been nice to have a test to make sure we don't regress in the future

[BenWiederhake]

You're right!