Add an SDL sound backend and use it instead of spawning an external

Add an SDL sound backend and use it instead of spawning an external
player.  SDL and SDL_mixer are now required.  You may get them from:

http://www.libsdl.org
http://www.libsdl.org/projects/SDL_mixer/

If you have problems building, try uncommenting the extra SDLLIBS and
SDLINCLUDES variables in src/sim/makefile.