Process.exitCode returns 0 when called again on POSIX #114

After the first successful waitpid, later exitCode() calls get ECHILD and return 0, so polling with exitCode(false) and then calling exitCode() loses the exit code (sh -c "exit 3" reports 3, then 0; Windows reports 3 both times). kill() and close() also act on a pid that may already have been reaped and reused. The status should be kept after the first reap.