fs: add a temporary fix for re-evaluation support

This is needed to give users a grace period before actually breaking
modules that re-evaluate fs sources from context where internal modules
are not allowed, e.g. older version of graceful-fs module.

To be reverted in Node.js 7.0

Fixes: #5097, see also #1898, #2026, and #4525.
PR-URL: https://github.com/nodejs/node/pull/5102


Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>