kill: support multiple signals for --list

[m-haisham]

Issue #6202

Broken support for querying multiple signal names

$ ../gnu/src/kill --version | head -n1
kill (GNU coreutils) 9.5
$ ../gnu/src/kill -l SIGUSR1 SIGUSR2
10
12
$ cargo run --all-features kill -l SIGUSR1 SIGUSR2
10
$

Changes

Multiple signal input support for --list command

$ cargo run --all-features kill -l SIGUSR1 SIGUSR2
10
12

Don't terminate and display error inline when signal is not recognized

$ cargo run --all-features kill -l SIGUSR3 SIGUSR2
kill: unknown signal name 'SIGUSR3'
12

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/tail/inotify-dir-recreate (passes in this run but fails in the 'main' branch)

[BenWiederhake]

Looks good, especially good catch regarding the continue-on-error behavior!

Please also write tests so that we don't regress in the future. (Eventually the argument parser will be rewritten, and it would be nice to know for sure that features like this didn't accidentally break.)

[m-haisham]

I have added necessary tests for multiple signals as well as to test for vertical list #6213

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/tail/inotify-dir-recreate (passes in this run but fails in the 'main' branch)

[BenWiederhake]

Loogs good, and thank you for adding the test for vertical signals!