src: add process.binding('config')

It turns out that userland likes to override process.config with
their own stuff. If we want to be able to depend on it in any way,
we need our own internal mechanism.

This adds a new private process.binding('config') that is
intended to serve as a container for internal flags and compile
time configs that need to be passed on to the JS layer.

PR-URL: https://github.com/nodejs/node/pull/6266


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>