clonesetup: add support to clone devices via /etc/clonetab

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

meson.build

@@ -302,6 +302,7 @@ conf.set_quoted('SYSTEMD_USERWORK_PATH',                      libexecdir / 'syst
 conf.set_quoted('SYSTEMD_MOUNTWORK_PATH',                     libexecdir / 'systemd-mountwork')
 conf.set_quoted('SYSTEMD_NSRESOURCEWORK_PATH',                libexecdir / 'systemd-nsresourcework')
 conf.set_quoted('SYSTEMD_VERITYSETUP_PATH',                   libexecdir / 'systemd-veritysetup')
+conf.set_quoted('SYSTEMD_CLONESETUP_PATH',                    libexecdir / 'systemd-clonesetup')
 conf.set_quoted('SYSTEM_CONFIG_UNIT_DIR',                     pkgsysconfdir / 'system')
 conf.set_quoted('SYSTEM_DATA_UNIT_DIR',                       systemunitdir)
 conf.set_quoted('SYSTEM_ENV_GENERATOR_DIR',                   systemenvgeneratordir)
@@ -2141,6 +2142,7 @@ subdir('src/bootctl')
 subdir('src/busctl')
 subdir('src/cgls')
 subdir('src/cgtop')
+subdir('src/clonesetup')
 subdir('src/coredump')
 subdir('src/creds')
 subdir('src/cryptenroll')