CBufferedFile: convert into a non-refcounted RAII wrapper

- it now takes over the passed file descriptor and closes it in the
  destructor
- this fixes a leak in LoadExternalBlockFile(), where an exception could
  cause the file to not getting closed

- disallow copies (like recently added for CAutoFile)
- make nType and nVersion private

src/serialize.h

@@ -1256,13 +1256,19 @@ public:
     }
 };
 
-/** Wrapper around a FILE* that implements a ring buffer to
- *  deserialize from. It guarantees the ability to rewind
- *  a given number of bytes. */
+/** Non-refcounted RAII wrapper around a FILE* that implements a ring buffer to
+ *  deserialize from. It guarantees the ability to rewind a given number of bytes. */
 class CBufferedFile
 {
 private:
-    FILE *src;          // source file
+    // Disallow copies
+    CBufferedFile(const CBufferedFile&);
+    CBufferedFile& operator=(const CBufferedFile&);
+
+    int nType;
+    int nVersion;
+
+    FILE *src;            // source file
     uint64_t nSrcPos;     // how many bytes have been read from source
     uint64_t nReadPos;    // how many bytes have been read from this
     uint64_t nReadLimit;  // up to which position we're allowed to read
@@ -1289,12 +1295,18 @@ protected:
     }
 
 public:
-    int nType;
-    int nVersion;
-
     CBufferedFile(FILE *fileIn, uint64_t nBufSize, uint64_t nRewindIn, int nTypeIn, int nVersionIn) :
-        src(fileIn), nSrcPos(0), nReadPos(0), nReadLimit((uint64_t)(-1)), nRewind(nRewindIn), vchBuf(nBufSize, 0),
-        nType(nTypeIn), nVersion(nVersionIn) {
+        nSrcPos(0), nReadPos(0), nReadLimit((uint64_t)(-1)), nRewind(nRewindIn), vchBuf(nBufSize, 0)
+    {
+        src = fileIn;
+        nType = nTypeIn;
+        nVersion = nVersionIn;
+    }
+
+    ~CBufferedFile()
+    {
+        if (src)
+            fclose(src);
     }
 
     // check whether we're at the end of the source file