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
