pdagentd.py should be able to recover from a stale pid file

[Brianna Bennett]

If there's an existing pid file, pdagentd.py just prints a message and quits, regardless whether there's an actual process running or not.

Instead, pdagentd.py when starting, if a pid file exists, it should check for the existence of that process. If there is no process running it should simply clean up the stale pid and move on.