Added support for Tandy 1000 HX and SL/2. As part of this commit :

- FDC now selects drive number from DOR (0x3f2)
- FDC now fails seeks on drives > 1
- XT systems no longer have BIOS mapping at E0000-EFFFF
- SN76489 now distinguishes between SN76496 (PCjr and early Tandy 1000), NCR8496 (later Tandy 1000 - different noise handling) and PSSJ
- Removed redundant limits on memory configuration - allows selection of 768k for Tandy 1000 SL/2

src/fdc.h

@@ -1,6 +1,7 @@
 void fdc_init();
 void fdc_add();
 void fdc_add_pcjr();
+void fdc_add_tandy();
 void fdc_remove();
 void fdc_reset();
 void fdc_poll();