p.conf: add flags-start-jessie, flags-end-jessie variables

Signed-off-by: Andreas Beckmann <anbe@debian.org>
scripts-jessie: make ucf fake-essential

the few buggy packages haven't been fixed for jessie either :-(

Signed-off-by: Andreas Beckmann <anbe@debian.org>
scripts-jessie: make init-system-helpers fake-essential

to allow proper cleanup of systemd service symlinks

Signed-off-by: Andreas Beckmann <anbe@debian.org>
scripts-leftovers: handle fake essential packages consistently

... like in the other scripts

Signed-off-by: Andreas Beckmann <anbe@debian.org>
p.conf: enable --warn-on-debsums-errors for wheezy2jessie upgrades

mostly for the benefit of the apache stack due to renaming conffiles and
moving them to a different package at the same time

Signed-off-by: Andreas Beckmann <anbe@debian.org>