Add an `always_off` option to outputs that causes them to never start
playback even if they're enabled.
This allows placeholder `null` outputs to be defined for the purpose
of having an external client react to the enabled state without the
side effects of real outputs. Like an external mixer, the client can
perform some action when an output is enabled.
Normally `null` outputs can be used for playback so it's possible for
MPD to continue playback silently if a problem occurs with all the real
outputs (or there are none enabled).
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