Fixes to sceAudio Reserve funcs

This does only a few things:
 * Validates parameters to sceAudioOutput2Reserve / sceAudioChReserve / sceAudioSRCChReserve.
 * Uses a separate channel fro SRC / Output2, since tests show they can all be reserved (especially 0) and SRC/Output2 can still work.
 * Reserves the highest channel first when doing automatic (could be important in some games...?)
 * Sets the channel's format to STEREO for SRC and Output2.  Wasn't initialized, but I guess STEREO is zero anyway.... I was initially worried this was a problem.

Mostly fixes tests, but the frequency validation isn't correct yet in SRC.

I tried a bunch of games that play sounds and call these functions, and they seemed unaffected.  I'm hoping these changes might help some game, though.  Making blocking audio work right is my real goal, but I wanted to understand these first to write proper tests.

-[Unknown]