Refactor the way image extensions are handled to be generally cleaner. Add

support for validating ImageSpecs before we actually want to read/write an
image, so as to allow us to check the extension *before* wasting time reading a
disk. Make .d81 an alias of .img.

lib/globals.h

@@ -22,7 +22,7 @@ extern void hexdumpForSrp16(std::ostream& stream, const Bytes& bytes);
 class Error
 {
 public:
-    ~Error()
+    [[ noreturn ]] ~Error()
     {
         std::cerr << "Error: " << _stream.str() << std::endl;
         exit(1);