Move all SDL backend function to the SDL backend and use SDL_ as prefix
Move all generic function used only by the SDL backend into it

Additionally rename those function to SDL_ prefix and refactor
them a little.
Retire snd_mix.c and incorporate it into the SDL backend
Retire snd_mem.c and integrate it into snd_dma.c and the backends
Print the number of OpenAL sources in the backend
Refactor the SDL part of S_LoadSound into the SDL backend
Refactor the SDL part of S_RawSamples into the backend
Refactor the SDL part of S_Update() into the backend
yq2 never had a dma based soundengine. Let's call it just "sound".
Update file header and comment
Whitespace cleanup
Cleanup local.h of the sound system
Cleanup snd_dma.c
Update header comment
Whitespace cleanup
Rename snd_dma.c to sound.c. yq2 never used DMA.
Restructure and refactor snd_al.c
Rework volume handling
Update header comment
Whitespace cleanup