dns: detect more quickly when systemd-resolved can't be D-Bus activated
Pass a callback and a 4-second timeout to the "StartServiceByName()" D-Bus call, so that we can detect any failure immediately.
In this way when systemd-resolved fails to start at boot (for example because it's masked), nm-online doesn't need to wait those additional 4 seconds due to the fixed timeout source.
Fixes-test: @nm_online_wait_for_delayed_device