Add DosExitCode to DosError shl 16 to set ExitCode value in utils/dosbox/exitcode.pas program

tests/utils/dosbox/exitcode.pas

@@ -23,7 +23,7 @@ begin
   SwapVectors;
 
   if DosError <> 0 then
-    ExitCode := DosError shl 16
+    ExitCode := DosError shl 16 + DosExitCode
   else
     ExitCode := DosExitCode;