Revisit the safety considerations of `rustix::io::stdout` and friends.
The Rust standard library assumes that the stdin, stdout, and stderr
file descriptors are always open, to the point where it's even willing
to hand out a [`'static` borrow for them]. Consequently, rustix doesn't
need to treat these as unsafe when std is in use.

[`'static` borrow for them]: https://doc.rust-lang.org/stable/std/io/struct.Stdin.html#method.lock