On Unix we have to be careful to not call `waitpid` twice, but we don't have to
be careful on Windows due to the way process handles work there. As a result the
cached `Option<ExitStatus>` is only necessary on Unix, and it's also just an
implementation detail of the Unix module.
At the same time. also update some code in `kill` on Unix to avoid a wonky
waitpid with WNOHANG. This was added in 0e190b9a to solve #13124, but the
`signal(0)` method is not supported any more so there's no need to for this
workaround. I believe that this is no longer necessary as it's not really doing
anything.