ossaudio: Make SNDCTL_DSP_[GET|SET][PLAY|RECORD]VOL closer to OSSv4

Problems in the previous code include returning values in the 0-255
range NetBSD uses instead of the 0-100 range OSSv4 expects, using
AUDIO_GETBUFINFO (which doesn't even return the mixer bits), and
not encoding channels as specified: "level=(left)|(right << 8)".

In reality, setting the gain in this way (through /dev/audio rather
than /dev/mixer) doesn't seem to work properly, and the mixer-set
value seems to be retained.

However, these changes at least ensure that the return values are
correct and the balance is set correctly.

I've only found one application using this API (audio/audacious), and
OSSv4 support in it is currently disabled precisely because it breaks
when it attempts to set the track volume using it.

(nia)