src: miscellaneous cleanups for node_config

Includes a fix for setting the `icuDataDir` as a UTF8 string
rather than one byte. Previously, if the dir contained any
non-ascii characters they would be mangled. This won't cover
cases that involve paths with other character encodings
(thank you Linux).. but it will cover the most likely.

Other miscellaneous cleanups are included

PR-URL: https://github.com/nodejs/node/pull/14868


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>