restore daemon: setup backup system user and group
now required as we always enforce lock files to be owned by the
backup user, and the restore code uses such code indirectly as the
REST server module is reused from proxmox-backup-server. Once that is
refactored out we may do away such things, but until then we need to
have a somewhat complete system env.
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>