unix ExitStatus: Do not treat WIFSTOPPED as WIFSIGNALED

A unix wait status can contain, at least, exit statuses, termination
signals, and stop signals.

WTERMSIG is only valid if WIFSIGNALED.

https://pubs.opengroup.org/onlinepubs/9699919799/functions/wait.html

It will not be easy to experience this bug with `Command`, because
that doesn't pass WUNTRACED.  But you could make an ExitStatus
containing, say, a WIFSTOPPED, from a call to one of the libc wait
functions.

(In the WIFSTOPPED case, there is WSTOPSIG.  But a stop signal is
encoded differently to a termination signal, so WTERMSIG and WSTOPSIG
are by no means the same.)

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

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

@@ -479,7 +479,7 @@ impl ExitStatus {
     }
 
     pub fn signal(&self) -> Option<i32> {
-        if !self.exited() { Some(libc::WTERMSIG(self.0)) } else { None }
+        if libc::WIFSIGNALED(self.0) { Some(libc::WTERMSIG(self.0)) } else { None }
     }
 }