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/fdd.h

@@ -5,6 +5,8 @@ int fdd_getrpm(int drive);
 void fdd_set_densel(int densel);
 int fdd_can_read_medium(int drive);
 int fdd_doublestep_40(int drive);
+int fdd_is_525(int drive);
+int fdd_is_ed(int drive);
 
 void fdd_set_type(int drive, int type);
 int fdd_get_type(int drive);