process::unix: Handle other wait statuses in ExitStatus as Display

Currently, on Nightly, this panics:

```
use std::process::ExitStatus;
use std::os::unix::process::ExitStatusExt;

fn main() {
    let st = ExitStatus::from_raw(0x007f);
    println!("st = {}", st);
}
```

This is because the impl of Display assumes that if .code() is None,
.signal() must be Some.  That was a false assumption, although it was
true with buggy code before
  5b1316f78152a9c066b357ea9addf803d48e114a
  unix ExitStatus: Do not treat WIFSTOPPED as WIFSIGNALED

This is not likely to have affected many people in practice, because
`Command` will never produce such a wait status (`ExitStatus`).

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>

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