Add sound toggle to options menu

[techknight]

I've been thinking about the way that toggling the sound is handled. If there's interest, I'd like to explore making these changes:

1. Change the toggle sound key from "S" to Ctrl+S
   • This frees the S key to be assignable for gameplay in the key bindings without losing the ability to toggle sound
   • Sound can then be toggled during the main menu, where "S" currently selects single player mode
2. Save the sound setting in sopwith.cfg and add it to the options menu
3. When the game is launched with the -q "quiet" option it will also change the sound setting to off. (There could also be a new command-line argument to force sound on, if desired.)