Adds dm-clone device setup at boot via a new /etc/clonetab config file, following the crypttab/veritytab pattern. - Add systemd-clonesetup-generator to parse /etc/clonetab and generate units. - Add systemd-clonesetup binary to create/remove dm-clone devices via ioctl. - Add clonesetup.target for ordering dm-clone activation at boot. - Add region_size= option in clonetab to configure dm-clone hydration granularity. - Add clonetab(5) and systemd-clonesetup-generator(8) man pages. Fixes: https://github.com/systemd/systemd/issues/39500