PIT: add alternative faster PIT

This is enabled by default on 486+ CPUs and can be forced disabled/enabled with pit_mode=0/1

src/86box.c

@@ -182,6 +182,7 @@ int	confirm_reset = 1;			/* (C) enable reset confirmation */
 int confirm_exit = 1;				/* (C) enable exit confirmation */
 int confirm_save = 1;				/* (C) enable save confirmation */
 int	enable_discord = 0;			/* (C) enable Discord integration */
+int pit_mode = -1;				/* (C) force setting PIT mode */
 
 /* Statistics. */
 extern int mmuflush;