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.

Command/RecvKey.hs

@@ -62,8 +62,7 @@ start key = fieldTransfer Download key $ \_p ->
 		oksize <- case Types.Key.keySize key of
 		        Nothing -> return True
 		        Just size -> do
-				size' <- fromIntegral . fileSize
-					<$> liftIO (getFileStatus tmp)
+				size' <- liftIO $ getFileSize tmp
 				return $ size == size'
 		if oksize
 			then case Backend.maybeLookupBackendName (Types.Key.keyBackendName key) of