impl Default for ExitCode
As suggested here
https://github.com/rust-lang/rust/pull/106425#issuecomment-1382952598
Needs FCP since this is an insta-stable impl.
Ideally we would have `impl From<ExitCode> for ExitStatus` and implement the default `ExitStatus` using that. That is sadly not so easy because of the various strange confusions about `ExitCode` (unix: exit status) vs `ExitStatus` (unix: wait status) in the not-really-unix platforms in `library//src/sys/unix/process`. I'll try to follow that up.