Add s6 backend

[cnt0]

s6 homepage: https://skarnet.org/software/s6/

I've been using this backend for about a week and can confirm that it works

@skarnet might want to review that as well

[skarnet]

Nitpick: if you run s6-svscan on $2, then $2 shouldn't be named a service directory, but a scan directory (which contains [links to] service directories), which is where you host a full supervision tree. A service directory is one that is managed by s6-supervise and contains the information for one service only.

Apart from that, I don't know the details of the turnstile API so I can't tell, but it looks okay.

As with anything involving user services, however, note that there's a question with logging: here, whatever user services write to their stdout and stderr will fall through to s6-svscan's stdout/stderr, i.e. whatever turnstile runs the script with. If these streams end up in a generic root log, be aware that a user can flood these logs, which is probably not a good idea.