We currently have issues with OpenVPN hook scripts that aren't always killed
at exit. Such orphan processes prevent re6st from starting again (EADDRINUSE).

We want to know if it's an OpenVPN that does not exit cleanly on TERM,
or if it sometimes does not exit at all after 5s (then re6st sends a KILL
signal and at that point we should indeed make sure that any subprocess is
also KILLed).