Reverse of init, step-by-step: systemctl disable --now decnet.target,
remove every decnet-*.service + decnet.target unit file, drop the
polkit rule, drop the tmpfiles.d entry, daemon-reload, remove
/etc/decnet + /etc/decnet/config.ini, /run/decnet, /opt/decnet, and
userdel/groupdel the decnet identity.
Preserves /var/lib/decnet and /var/log/decnet by default — those
hold operator data. Pass `--deinit --purge` to rm -rf them too.
Idempotent on a clean host (every step prints [SKIP]). Honours
--dry-run.
5 new tests cover the full-undo path, --purge, idempotent clean-host
deinit, dry-run side-effect-free behaviour, and the --purge without
--deinit guard.