debian: migrate legacy mac/ipam cache/state files

debian: migrate legacy mac/ipam cache/state files

We opted for a simple copy of the legacy file to the new location,
since any other migration path would be too strenuous. The worst-case
scenario of failures with this approach is losing IPAM / MAC entries
that are created during the update window in the legacy files by nodes
that are not yet updated to the new version. Those can be fixed by a
simple start / stop of the affected VMs, triggering a rewrite of the
IPAM database.

Signed-off-by: Stefan Hanreich <s.hanreich@proxmox.com>
 [ TL: output message if files get moved and add todo-comment ]
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>