[kill] Implement kill -l EXIT_STATUS

[duckinator]

I've implemented kill -l but not kill -l EXIT_STATUS

Frankly, I don't understand what https://pubs.opengroup.org/onlinepubs/9699919799/utilities/kill.html is trying to tell me:

-l
(The letter ell.) Write all values of signal_name supported by the implementation, if no operand is given.
If an exit_status operand is given and it is a value of the '?' shell special parameter (see Special Parameters and wait) corresponding to a process that was terminated by a signal, the signal_name corresponding to the signal that terminated the process shall be written. If an exit_status operand is given and it is the unsigned decimal integer value of a signal number, the signal_name (the symbolic constant name without the SIG prefix defined in the Base Definitions volume of POSIX.1-2017) corresponding to that signal shall be written.
Otherwise, the results are unspecified.

(Emphasis mine.)