Code review comment for lp:~jml/launchpad/sftp-poppy

Looks good. Two comments:

* It's not necessary to do waitpid(pid, os.WNOHANG) followed by
  kill(pid, 0); the return value from waitpid() will be (0, 0) when
  the process has not exited yet, and it will error the same as kill()
  if the process does not exist.

* It might be nice to be able to pass a subprocess.Popen into
  two_stage_kill, so that its poll() method is used, and so the
  returncode attribute is set correctly to avoid confusion. But,
  really, this is probably not worth the effort.