scripted-diff: Rename CBlockDiskPos to FlatFilePos.

-BEGIN VERIFY SCRIPT-
sed -i 's/CDiskBlockPos/FlatFilePos/g' $(git ls-files 'src/*.h' 'src/*.cpp')
-END VERIFY SCRIPT-

src/init.cpp

@@ -672,7 +672,7 @@ static void ThreadImport(std::vector<fs::path> vImportFiles)
     if (fReindex) {
         int nFile = 0;
         while (true) {
-            CDiskBlockPos pos(nFile, 0);
+            FlatFilePos pos(nFile, 0);
             if (!fs::exists(GetBlockPosFilename(pos)))
                 break; // No block files left to reindex
             FILE *file = OpenBlockFile(pos, true);