stream: fix osx select hack

After latest twiddling, `stream->io_watcher.fd` doesn't contain a real
stream's file descriptior anymore. The one from `select_state` should be
used instead.

Zero-initialize `select_state->event` field.

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@bnoordhuis, I'm really really sorry for this, but it really makes things work. (i.e. right now `node < /dev/tty` is fucked again. I promise you to eventually write test for this stuff, no matter how complicated it will be.