Our customized runit supports a systemd-like mode allowing init to exit
inside containers (and to be signaled to halt from outside, via
SIGRTMIN+3).  This is critically important for Docker containers, since
Docker will wait ~10 seconds before sending a final killall on
'docker restart'.  We were already building runit for rpm-based distros,
since it's not packaged by default for them, so this just takes it a
little further.  Now container death is nearly instantaneous.

(Also fix a couple stray runit config bugs in 1,2,3.)