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/vid_ps1_svga.h

@@ -0,0 +1 @@
+extern device_t ps1_m2121_svga_device;