Uses policy-rc.d to prevent dpkg starting daemons

Currently the dpkg element fakes start-stop-daemon, initctl and
invoke-rc.d to stop dpkg from starting a bunch of daemons in the
chroot. This is problematic when packages use service, start, stop or
restart commands.

This patch uses a policy-rc.d instead of faking invoke-rc.d to achieve
the same thing. This approach now aligns exactly with debootstrap.
Without this patch DIB runs on some debian distributions fail
when trying to umount the chroot loop device as there are daemons
running.

The log will now show "invoke-rc.d: policy-rc.d denied execution of
start." instead of "Warning: Fake invoke-rc.s called, doing nothing."

Change-Id: I6db192127aca19b5b7915179b781f5192078bfc7
Related-Bug: #1211564
Merge "Uses policy-rc.d to prevent dpkg starting daemons"