Replace the real gnome-backgrounds with a fake, equivs generated one (Closes: #8055).

Jessie's gnome-shell depends on gnome-backgrounds, which is too fat to ship
considering we're not using it.
Install Florence from Debian unstable (partly addresses #8010).
Convert two custom initscripts to native systemd unit files.

This fixes the udev-watchdog startup, and starts our migration to systemd.
Disable /etc/network/if-pre-up.d/macchanger (Closes: #8265).

On Jessie, macchanger ships /etc/network/if-pre-up.d/macchanger, that probably
conflicts with the way we're spoofing the MAC address. Let's disable it via
the variable meant for this in /etc/default/macchanger.
Point to the macchanger patch in the relevant design doc.
Make the files we ship in /lib/systemd world-readable.

Otherwise, systemd warns about it (if one has built the ISO with a strict
umask). No big deal, but no reason to hide these files, so let's clean
things up a bit.
Use socket activation only for CUPS, to save a bit of memory and boot time.
AppArmor: adjust CUPS profile to support our Live system environment (Closes: #8261).

Apart of the usual lib/live/mount/overlay/... mangling: the systemd unit file
uses PrivateTmp, so we need to use the attach_disconnected flag.

There are still quite some AppArmor denial logs, that I couldn't easily get rid
of, but at least the service starts, and I could "print" to a file.
Fix config file path for adjusting memlockd's OOM settings.
Fix permissions issue at tails-persistence-setup startup (Closes: #8097).

We got: "Can't locate Moose/Meta/Attribute/Custom/Trait/Counter.pm: Permission
denied at /usr/share/perl5/Module/Runtime.pm line 317", because "." is in @INC,
and apparently sudo keeps $PWD unchanged on Jessie.