src: don't use fopen() in require() fast path

Fix a regression that was introduced in commit 1bbf8d07 ("lib: speed up
require(), phase 2") where file paths with Unicode characters fail to
load on Windows.

Fixes: https://github.com/nodejs/node/issues/2236
PR-URL: https://github.com/nodejs/node/pull/2377


Reviewed-By: Bert Belder <bertbelder@gmail.com>