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>

library/std/src/sys/unix/process/process_unix.rs

@@ -527,9 +527,18 @@ impl fmt::Display for ExitStatus {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         if let Some(code) = self.code() {
             write!(f, "exit code: {}", code)
+        } else if let Some(signal) = self.signal() {
+            if self.core_dumped() {
+                write!(f, "signal: {} (core dumped)", signal)
+            } else {
+                write!(f, "signal: {}", signal)
+            }
+        } else if let Some(signal) = self.stopped_signal() {
+            write!(f, "stopped (not terminated) by signal: {}", signal)
+        } else if self.continued() {
+            write!(f, "continued (WIFCONTINUED)")
         } else {
-            let signal = self.signal().unwrap();
-            write!(f, "signal: {}", signal)
+            write!(f, "unrecognised wait status: {} {:#x}", self.0, self.0)
         }
     }
 }