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/mem.h

@@ -163,4 +163,6 @@ void mem_reset_page_blocks();
 
 extern mem_mapping_t ram_low_mapping;
 
+void mem_remap_top_384k();
+
 #endif