Revert "drop unused policy-disable-rc.d"

Revert "drop unused policy-disable-rc.d"

This reverts commit 8ee12a1c34f9b5980cb382aa2bb01855349a10a5.

`policy-rc.d` invocation is still used by deb-systemd-invoke (1p), despite
it's by now unfitting name.

The policy-rc.d script the installer placed into the chroot prevents the
starting of services by (debianized) maintainer-scripts [0].
This should reduce installation time on one hand, and on the other hand does
mask errors in maintainer-scripts, which error out if no systemd is running
(e.g. inside the chroot)

Noticed the problem, when testing an unrelated patch and running into
an aborted installtion, due to a, already fixed, glitch (missing '||true')
in pve-lxc's postinst script.

[0] https://people.debian.org/~hmh/invokerc.d-policyrc.d-specification.txt
   (did not find anything more current, contradicting the information there)

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>