Add emulation of IBM PS/1 Model 2121.

As part of this :
- FDC Specify command now completes immediately
- Implemented keyboard system flag status
- Added ability to remap 384kb reserved memory to above extended memory

src/fdc.h

@@ -9,6 +9,7 @@ void fdc_abort();
 void fdc_discchange_clear(int drive);
 void fdc_set_dskchg_activelow();
 void fdc_3f1_enable(int enable);
+void fdc_set_ps1();
 int fdc_get_bitcell_period();
 
 /* A few functions to communicate between Super I/O chips and the FDC. */