On Windows, strerror returns just "Unknown error" for errnum greater
than MAX_ERRNO, while linux and freebsd returns "Unknown error <num>",
which is the current expectation for errno_autotest. Differentiate
the error string on Windows to remove a "duplicate error code" failure.
Signed-off-by: Jie Zhou <jizh@linux.microsoft.com>
Acked-by: Dmitry Kozlyuk <dmitry.kozliuk@gmail.com>