Sentinel support

Was #452.
First issue: you're correct, also to avoid issues with gitignore it'll be better to just copy the example into separate files for each sentinel and gitignore those.

Second issue: I guess I could add a check to see if all the pids are alive, but in any case this is not intended to be a bullet-proof script for loading sentinels - rather, just a quick way to test stuff. I agree it should do this is a somewhat reliable way.