kill: list signal 0 with -l and -t

[cakebaker]

This PR shows signal 0 (EXIT) in the output of -l/--list and -t/--table. It's a change made in GNU coreutils 9.6:

kill -l and -t now list signal 0, as it's a valid signal to send.

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/misc/usage_vs_getopt. tests/misc/usage_vs_getopt is passing on 'main'. Maybe you have to rebase?
Congrats! The gnu test tests/misc/stdbuf is no longer failing!