Add API to control audio device parameters exposed by sndiod.

The API exposes controls of modern audio hardware and sndiod software
volume knobs in a uniform way. Hardware knobs are exposed through

Add API to control audio device parameters exposed by sndiod.

The API exposes controls of modern audio hardware and sndiod software
volume knobs in a uniform way. Hardware knobs are exposed through
sndiod. Multiple programs may use the controls at the same time
without the need to continuously scan the controls.

For now sndiod exposes only its own controls and the master output and
input volumes of the underlying hardware (if any), i.e. those
typically exposed by acpi volume keys.

ok deraadt

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