Remove extern global variable from bios_disk.h

This way interface of swapInDisks function is cleaner and we avoid a
warning when comparing (previously) signed swap position with an
unsigned array size or index.

Also, add some documentation to swapInDisks function.

src/dos/dos_programs.cpp

@@ -676,9 +676,7 @@ public:
 			i++;
 		}
 
-		swapPosition = 0;
-
-		swapInDisks();
+		swapInDisks(0);
 
 		if (!imageDiskList[drive - 'A']) {
 			WriteOut(MSG_Get("PROGRAM_BOOT_UNABLE"), drive);