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/main.cpp

@@ -3084,6 +3084,7 @@ bool LoadExternalBlockFile(FILE* fileIn, CDiskBlockPos *dbp)
 
     int nLoaded = 0;
     try {
+        // This takes over fileIn and calls fclose() on it in the CBufferedFile destructor
         CBufferedFile blkdat(fileIn, 2*MAX_BLOCK_SIZE, MAX_BLOCK_SIZE+8, SER_DISK, CLIENT_VERSION);
         uint64_t nStartByte = 0;
         if (dbp) {
@@ -3140,7 +3141,6 @@ bool LoadExternalBlockFile(FILE* fileIn, CDiskBlockPos *dbp)
                 LogPrintf("%s : Deserialize or I/O error - %s", __func__, e.what());
             }
         }
-        fclose(fileIn);
     } catch(std::runtime_error &e) {
         AbortNode(_("Error: system error: ") + e.what());
     }