Added policy-rc.d that turns invoke-rc.d into a no-op

The problem is that in older versions of Ubuntu, the unpack packageset from the target distribution to be able to run the first-stage debootstrap is not very minimal.

In particular, it includes packages like initscripts, udev and procps, which have initscripts that are started by postinst. In older Ubuntu, when a postinst runs invoke-rc.d, it communicates with Upstart via a mechanism that can escape from the chroot and talk to the host system's Upstart (!); and if it can't, invoke-rc.d will fail and the first-stage bootstrap will also fail.

The symptom is that on workers whose host OS did not boot using Upstart (for example Debian stretch, but not Ubuntu trusty), the first-stage bootstrap fails with:

[ 22s] initctl: Unable to connect to Upstart: Failed to connect to socket /com/ubuntu/upstart: Connection refused

So here is a patch to provide OBS-level solution that adds a policy-rc.d that turns invoke-rc.d into a no-op, just like debootstrap does to prevent processes in deb build chroots from trying to start system services.

Signed-off-by: Simon McVittie smcv@collabora.com Tested-by: Héctor Orón Martínez hector.oron@collabora.co.uk Signed-off-by: Andrew Lee (李健秋) ajqlee@debian.org