Fix nodefs windows

Fixes node.js native filesystem tests test_unistd_io, test_fs_nodefs_rw, test_unistd_access, test_unistd_links, test_unistd_misc and test_unistd_truncate on Windows. Closes #1715.

Currently running linux try build at http://clb.demon.fi:8112/builders/try-ubuntu/builds/10/steps/Ubuntu%20test%20emscripten%20try%20branch/logs/stdio and Windows try build at http://clb.demon.fi:8112/builders/try-win8/builds/13/steps/Test%20emscripten%20try%20branch/logs/stdio , if there's no regressions especially on the linux side, this will be good to go. I'll update this comment once try finishes.