the /usr/sbin/policy-rc.d script and returning 101, per
https://people.debian.org/~hmh/invokerc.d-policyrc.d-specification.txt,
but only for the window while we're installing Salt.
This is a much more fool-proof method than what I was attempting
before. I hunted for how to do this before and clearly failed at my
Google-fu.
Fixes#5621