Though the client shouldn't be calling anything when NM isn't running
(because clients have nm_client_get_manager_running()), make sure
that NMClient never calls a NetworkManager method when NM isn't
on the bus.

Next, ensure NMObject doesn't try to refresh properties when NM isn't
running.  Creating an NMClient may trigger a property refresh request,
but if NM isn't running, defer that until NM starts, to ensure that
we don't D-Bus autostart NM.

Third, ensure NMRemoteSettings doesn't attempt to list connections
unless NM is running.

This prevents service activation of NetworkManager in lieu of dbus-glib
learning about DBUS_HEADER_FLAG_NO_AUTO_START.