connectivity: avoid D-Bus activating systemd-resolved when we know it's not used
Every (failed) attempt to D-Bus activate a service results in log-messages from dbus-daemon. It must be avoided to spam the logs that way. Let connectivity check not only ask whether systemd-resolved is enabled (and NetworkManager would like to push information there), but also whether it looks like the service is actually available. That is, either it has a name-owner or it's not blocked from starting. The previous workaround was to configure main.systemd-resolved=no in NetworkManager.conf. But that requires explict configuration.
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