Windows: make stdout/sterr pipes blocking

See https://github.com/joyent/node/issues/3584

I took 0b3021ed3389436fe399fdbb02b0d65c23150592 and modified slightly
to avoid exposing the 'blocking' option. Instead, stdout and stderr
pipes are always implicitly set as blocking.

This fixes test-stream2-stderr-sync.js on Windows.