Add scePadIsSupportedAudioFunction, scePadSetAudioOutPath and
scePadSetVolumeGain. The path and the speaker, headset and microphone
gain are kept per port in the pad output state, where a host backend
can fetch them, and a repeated identical value does not bump the output
sequence.
A path above 4 is rejected with INVALID_ARG, a null gain with
INVALID_ARG, an unknown handle with INVALID_HANDLE, and everything
returns NOT_INITIALIZED before scePadInit. Special handles accept the
setters without effect. Signatures and path values follow duaLib; no
host backend routes controller audio yet, which the technical debt file
records.
