audio/sdl2_sound: add port: Abstract SDL soundfile decoder

- Add the latest version of sdl_sound with support for
  USE_SDL=sound2.
- Note that this is sdl2-only library, so audio/sdl_sound,
  which is sdl1-only library, remains in the tree and is
  not affected. I've had to rename playsound binary this
  port also installs to avoid the conflict.
- The library now contains all the decoder code, so no external
  depends and options are needed any longer. I've added MIDI
  option though, since upstream has it disabled by default due
  to different license.