Fixed some stupid serial issues - reading interrupt enable mask now works, and has an effect, and reading data when FIFO empty now returns last value instead of zero. Windows 9x setup now detects serial mouse correctly.

src/serial.h

@@ -8,6 +8,7 @@ typedef struct
 {
         uint8_t linestat,thr,mctrl,rcr,iir,ier,lcr;
         uint8_t dlab1,dlab2;
+        uint8_t dat;
 } SERIAL;
 
 extern SERIAL serial,serial2;