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/ibm.h

@@ -300,6 +300,8 @@ enum
         ROM_DTK386,
         ROM_PXXT,
         ROM_JUKOPC,
+        ROM_TANDY1000HX,
+        ROM_TANDY1000SL2,
         ROM_IBMAT,
         ROM_CMDPC30,
         ROM_AMI286,