fs: move stringToFlags() to lib/internal

PR-URL: https://github.com/nodejs/node/pull/7162
Refs: https://github.com/nodejs/node/pull/6413


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>