kill: add support for undocumented argument -n SIGNUM

[jfinkels]

Environment: Ubuntu 20.04, uutils main branch (git commit 88cdf16), GNU coreutils v9.5.227-28b1

Steps to reproduce:

env kill -n0 $$

$$ in bash is the PID of the current shell, according to https://www.gnu.org/software/bash/manual/html_node/Special-Parameters.html.

What I expected to happen: GNU coreutils kill sends the signal 0 to the process and terminates successfully. The -n argument is undocumented, but allowed for compatibility with the bash built-in command kill.

What happens now: uutils kill complains that the -n argument is unexpected:

error: unexpected argument '-n' found

  tip: to pass '-n' as a value, use '-- -n'

Usage: ./target/debug/kill [OPTIONS]... PID...

For more information, try '--help'.

Notes: this is causing a failure in the GNU test file tests/misc/kill.sh.