src: throw on process.env symbol usage

This commit causes process.env to throw when a symbol is used as
either a key or a value.

Fixes: https://github.com/nodejs/node/issues/9429
PR-URL: https://github.com/nodejs/node/pull/9446


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Minwoo Jung <jmwsoft@gmail.com>