add getFileSize, which can get the real size of a large file on Windows

Avoid using fileSize which maxes out at just 2 gb on Windows.
Instead, use hFileSize, which doesn't have a bounded size.
Fixes support for files > 2 gb on Windows.

Note that the InodeCache code only needs to compare a file size,
so it doesn't matter it the file size wraps. So it has been
left as-is. This was necessary both to avoid invalidating existing inode
caches, and because the code passed FileStatus around and would have become
more expensive if it called getFileSize.

This commit was sponsored by Christian Dietrich.

Remote/Helper/Chunked.hs

@@ -244,8 +244,7 @@ retrieveChunks retriever u chunkconfig encryptor basek dest basep sink
 	| otherwise = go =<< chunkKeys u chunkconfig basek
   where
 	go ls = do
-		currsize <- liftIO $ catchMaybeIO $
-			toInteger . fileSize <$> getFileStatus dest
+		currsize <- liftIO $ catchMaybeIO $ getFileSize dest
 		let ls' = maybe ls (setupResume ls) currsize
 		if any null ls'
 			then return True -- dest is already complete