core: add [State|Runtime|Cache|Logs]Directory symlink as second parameter

When combined with a tmpfs on /run or /var/lib, allows to create
arbitrary and ephemeral symlinks for StateDirectory or RuntimeDirectory.
This is especially useful when sharing these directories between
different services, to make the same state/runtime directory 'backend'
appear as different names to each service, so that they can be added/removed
to a sharing agreement transparently, without code changes.

An example (simplified, but real) use case:

foo.service:
StateDirectory=foo

bar.service:
StateDirectory=bar

foo.service.d/shared.conf:
StateDirectory=
StateDirectory=shared:foo

bar.service.d/shared.conf:
StateDirectory=
StateDirectory=shared:bar

foo and bar use respectively /var/lib/foo and /var/lib/bar. Then
the orchestration layer decides to stop this sharing, the drop-in
can be removed. The services won't need any update and will keep
working and being able to store state, transparently.

To keep backward compatibility, new DBUS messages are added.

src/systemctl/systemctl-show.c

@@ -1773,6 +1773,24 @@ static int print_property(const char *name, const char *expected_value, sd_bus_m
                         if (r < 0)
                                 return bus_log_parse_error(r);
 
+                        return 1;
+                } else if (STR_IN_SET(name, "StateDirectorySymlink", "RuntimeDirectorySymlink", "CacheDirectorySymlink", "LogsDirectorySymlink")) {
+                        const char *a, *p;
+                        uint64_t symlink_flags;
+
+                        r = sd_bus_message_enter_container(m, SD_BUS_TYPE_ARRAY, "(sst)");
+                        if (r < 0)
+                                return bus_log_parse_error(r);
+
+                        while ((r = sd_bus_message_read(m, "(sst)", &a, &p, &symlink_flags)) > 0)
+                                bus_print_property_valuef(name, expected_value, flags, "%s:%s", a, p);
+                        if (r < 0)
+                                return bus_log_parse_error(r);
+
+                        r = sd_bus_message_exit_container(m);
+                        if (r < 0)
+                                return bus_log_parse_error(r);
+
                         return 1;
                 }