PIT timers can now be restarted using GATE input - CPU speed detection on SiS496/497 now works.

Added missing port 0x61 to Olivetti M24 - PC speaker now works on that machine.
Some code tidying.

src/ibm.h

@@ -198,6 +198,7 @@ typedef struct PIT
         int thit[3];
         int delay[3];
         int rereadlatch[3];
+        int gate[3];
 } PIT;
 
 PIT pit;