This change has the insight that dwExitCode isn't an exit code but
rather should be used to pass the wait status. This lets us report
killing as a termination status, similar to UNIX. This change also
fixes the fact that exit(259) on Windows will break the parent due
way WIN32 is designed. We now work around that.
It turns out that NetBSD and OpenBSD, will let you have exit codes
beyond 255. This change will let you use them when it's possible.
