fixes for launchd, plist, virtualenvs, cwd

* Hardcode interpreter path in plist and nm scripts. This way they'll
  run in any virtualenv you might be using.
* Add --sleep option and use that in --plist, because launchd gets upset
  if execution takes less than 10 seconds.
* Move log path for plist so permissions issues are less likely.
* Delay getting current IP a bit on ddns updates, to avoid error where
  network isn't completely up. I may have to add a pre-sleep if it's
  still an occasional problem.
* Resolve symlinks when getting path to cfdns