[o11c]

I thought for sure this was wrong, but when I actually checked the docs, it turns out that `RWF_NOWAIT` is only valid for `preadv2` not `pwritev2`. This should probably be fixed.

For sockets, `MSG_DONTWAIT` works with both `recv` and `send`.

For pipes you should be able to do this with `SPLICE_F_NONBLOCK` and the `splice` family, but there are weird restrictions for those.