unix: impl ExitStatusExt for ExitStatusError

unix: impl ExitStatusExt for ExitStatusError

It is unergnomic to have to say things like
   bad.into_status().signal()

Implementing `ExitStatusExt` for `ExitStatusError` fixes this.
Unfortunately it does mean making a previously-infallible method
capable of panicing, although of course the existing impl remains
infallible.

The alternative would be a whole new `ExitStatusErrorExt` trait.

`<ExitStatus as ExitStatusExt>::into_raw()` is not particularly
ergonomic to call because of the often-required type annotation.
See for example the code in the test case in
  library/std/src/sys/unix/process/process_unix/tests.rs

Perhaps we should provide equivalent free functions for `ExitStatus`
and `ExitStatusExt` in std::os::unix::process and maybe deprecate this
trait method.  But I think that is for the future.

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