fs: don't alter user provided `options` object

This patch makes a copy of the `options` object before the fs module
functions alter it.

PR-URL: https://github.com/nodejs/node/pull/7831
Fixes: https://github.com/nodejs/node/pull/7655


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Nicu Micleușanu <micnic90@gmail.com>
Reviewed-By: Rod Vagg <rod@vagg.org>