libnative: process spawning should not close inherited file descriptors

Retry pull requesting of https://github.com/rust-lang/rust/pull/16407 after accidentally messing up rebasing of branches and making bors think the PR was merged
Could you add this test to `std::io::process` instead? The `iotest!` macro will allow the test to be run under both libnative and libgreen (to ensure consistency)