add audout:a audout:d aud:a aud:d

[ITotalJustice]

Application use: I use this in sys-tune to allow for per title volume control whilst keeping the master volume the same.

I've tried to match the style of audout.c/h. It is lacking comments, but i think the function names are descriptive enough.

I have a wrapper for common functions as well, but that probably doesn't belong in libnx :)