lib: allow process kill by signal number

This brings the behaviour in line with the documentation.

PR-URL: https://github.com/nodejs/node/pull/16944


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>