Sound refactor backport

• Splitscreen sound handling code is much more consistent now
  • Loops over splitscreen players instead of rancid code duplication.
  • Only plays a sound once, and instead handles volume based on the player object/viewpoint closest to it.
  • Handles splitscreen global volume reduction in one place, instead of duplicated across multiple functions
  • Resolves #22 (closed)
• Let S_StopSound functions stop multiple sounds at once