Add new utility functions FileCommit(), RenameOver()

src/main.h

@@ -956,13 +956,7 @@ public:
         // Flush stdio buffers and commit to disk before returning
         fflush(fileout);
         if (!IsInitialBlockDownload() || (nBestHeight+1) % 500 == 0)
-        {
-#ifdef WIN32
-            _commit(_fileno(fileout));
-#else
-            fsync(fileno(fileout));
-#endif
-        }
+            FileCommit(fileout);
 
         return true;
     }