Updates to SDL_sound.

[Ryan C. Gordon]

After hours of tracking down a bogus pointer, we've now got a Shorten
(.SHN) decoder in place, so you can listen to all those bootlegged Phish
and Grateful Dead shows on the 'net.  :)

Max: I changed the decoders to include config.h if needed, since config.h
now handles defining SOUND_SUPPORTS_*, which means the decoders won't
compile unless this header is included; it's a catch 22, y'know...

Let me know if I screwed it up.

--ryan.