Draft: libvirt-daemon-system: Reintroduce sysv init support

Get rid of the libvirt-daemon-system-systemd and libvirt-daemon-system-sysv packages, or more accurately turn them into dummy packages that can be dropped in bookworm+1.

Unfortunately sysv init scripts are considered conffiles, so we can't move them around without some additional care. Luckily, the code that we have used to transfer conffiles between packages in the past is battle-tested and works perfectly fine in this scenario.

The libvirt-daemon-system gains a dependency against systemd-container | sysvinit-core, which ensures that systemd-container is installed on systemd based hosts while at the same time leaving sysv init based hosts alone. lsb-core is also back as a dependency, as was the case before d88536dc.

I have performed several upgrades from bullseye to testing with these changes included. In all cases, there was at least one VM running, to ensure that all services and sockets would be up. The init system on the various test environments was either systemd, sysv-rc or openrc. All upgrades completed smoothly, with no failures being reported and the VM still running and perfectly functional afterwards.

Note that the commits from !165 (merged), !167 (merged) and !168 (merged) are included for convenience. Only the topmost commit is really supposed to be part of this MR.