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

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