resolved: actually unregister DNS-SD services when the D-Bus client vanishes (#43578)

Since the D-Bus DNS-SD API was introduced in c3036641f0ed (v236),
`RegisterService()` has created an `sd_bus_track` object to destroy the
registered service when the registering client disconnects from the bus.
However, the track object is a `_cleanup_(sd_bus_track_unrefp)` local in
`bus_method_register_service()` that is never stored anywhere, so it is
destroyed again as soon as the method handler returns. The vanish
handler (`dnssd_registered_service_on_bus_track()`) has thus been dead
code for the past ~8 years.

The observable effect: services registered via D-Bus silently persist
after the client disconnects — unowned, advertised until resolved exits,
and impossible to re-register since the identifier stays taken
(`DNSSD_SERVICE_EXISTS`). At the same time, the registration doesn't
survive a resolved restart, so the current behavior is neither reliably
ephemeral nor reliably persistent. Noticed while reviewing a project
that (unknowingly relying on this bug) registers a service with a
one-shot `gdbus call` and expects it to persist.

This PR:
1. splits the goodbye/zone-removal logic out of
`bus_dnssd_method_unregister()` into a helper — the vanish path needs
the full teardown, not just `dnssd_registered_service_free()`, which
would leave the service's RRs published in the per-link mDNS zones;
2. stores the track object in `DnssdRegisteredService`, making client
tracking work as originally intended;
3. documents the lifetime semantics in `org.freedesktop.resolve1(5)`
(matching the equivalent Avahi D-Bus API, whose entry groups are
likewise bound to the client connection) and points to `*.dnssd` files
for registrations independent of a client's lifetime;
4. adds a TEST-75 case covering the connection-bound lifetime.

Note this is a behavioral change for fire-and-forget clients (one-shot
`busctl`/`gdbus` invocations). Arguably those were never working
reliably (registrations silently vanished on any resolved restart, and
stale entries blocked re-registration), but if this is deemed too risky,
the alternative would be to drop the dead tracking code and codify the
persist-until-exit behavior instead — I'd argue tracking as originally
intended is the better contract.

Verification: code compiles; the new integration test is included.

NEWS

@@ -4,6 +4,19 @@ CHANGES WITH 262 in spe:
 
         Feature Removals and Incompatible Changes:
 
+        * DNS-SD services registered via systemd-resolved's D-Bus method
+          org.freedesktop.resolve1.Manager.RegisterService() are now
+          unregistered automatically when the client that registered them
+          disconnects from the bus, as originally intended and matching the
+          behavior of the equivalent Avahi API. Due to a bug the client
+          tracking never worked, and such services used to stay registered
+          until systemd-resolved exited (but did not survive a restart of the
+          daemon either). Clients must now keep their bus connection open for
+          as long as the service shall remain registered; one-shot
+          registrations, e.g. via busctl or gdbus, will be withdrawn again as
+          soon as the tool exits. To register a DNS-SD service independently
+          of a client's lifetime, use *.dnssd files, see systemd.dnssd(5).
+
         * Meson options '-Dlibiptc=' (deprecated in v259), '-Dlibidn=',
           '-Drc-local=', '-Dsysvinit-path=', and '-Dsysvrcnd-path=' (deprecated
           in v260) have been removed.