kill: accept all cases for signal names

[m-haisham]

This PR updates the signal name comparison to work with all cases. The changes are effective on --list and --signal commands.

$ cargo run --features kill kill -l sIgUsR2
12
[$? = 1]
$ kill -l sIgUsR2 # bash built-in
12
$ /usr/bin/kill -l sIgUsR2 # Debian procps-ng 4.0.4
12

Fixes #6217

[github-actions]

GNU testsuite comparison:

Congrats! The gnu test tests/timeout/timeout is no longer failing!

[BenWiederhake]

Does exactly what it says on the tin, and thorough tests. Thank you! :D