util: move .decorateErrorStack to internal/util

Move the method that was added in commit 8ca412b7 from earlier this month
from lib/util.js to lib/internal/util.js.

Avoids exposing a method that we may not wish to expose just yet, seeing
how it relies on implementation details.

PR-URL: https://github.com/nodejs/node/pull/4026


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>