Added --nostereo option

Ability to disable stereo panning effect (e.g., paint brush, UI
elements sound effect feedback, etc.), useful for users with
hearing impairment in one ear, or situations where one speaker or
headphone is being used.  Use "--nostereo" command-line option
or "nostereo=yes" config. file option.

src/parse.h

@@ -54,6 +54,7 @@ struct cfginfo
   const char *start_blank;
   const char *use_print_config;
   const char *use_sound;
+  const char *use_stereo;
   const char *wheely;
   const char *mouseaccessibility;
   const char *onscreen_keyboard;