file_io, DiskImage, sharpmz: Replace fd with FILE*

Replace file descriptor with a file stream in preparation for supporting
zipped files.

The API to miniz requires the use of file streams and since a file
descriptor can easily be extracted from a stream using `fileno` it's
much more convenient to use file streams everywhere.

DiskImage.h

@@ -80,7 +80,7 @@ public:
 
    void Open(const char *filename, bool ReadOnly);
 
-   void writeTRD(int hfile);
+   void writeTRD(fileTYPE *hfile);
 
    void readSCL(int hfile, bool readonly);
    void readFDI(int hfile, bool readonly);
@@ -189,4 +189,4 @@ int x2trd(const char *name, fileTYPE *f);
 int x2trd_ext_supp(const char *name);
 
 //-----------------------------------------------------------------------------
-#endif
+#endif