lib,src: audit process.env in lib/ for setuid binary

Wrap SafeGetenv() in util binding with the purpose of protecting
the cases when env vars are accessed with the privileges of another
user in jsland.

PR-URL: https://github.com/nodejs/node/pull/18511
Fixes: https://github.com/nodejs/node/issues/9160


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>