S6 is a process supervision suite, like daemontools (skarnet.org)

This definitely reminds me of DJB's supervise. They specifically mention it. I see a lot of patches to DJB software on his page.. looks like whoever wrote this is a fan of the DJB style, as am I. Very neat. Haven't thought about DJB in a long time.

Edit: Ahh, I found this linked from his page. It all makes perfect sense now. https://skarnet.org/software/skalibs/djblegacy.html