free space checking

Free space checking is now done, for transfers of data for keys that have free space metadata.
(Notably, not for SHA* keys generated with git-annex 0.24 or earlier.)

The code is believed to work on Linux, FreeBSD, and OSX; check compile-time
messages to see if it is not enabled for your OS.

Command/Migrate.hs

@@ -55,7 +55,7 @@ perform file oldkey newbackend = do
 	case stored of
 		Nothing -> return Nothing
 		Just (newkey, _) -> do
-			ok <- getViaTmp newkey $ \t -> do
+			ok <- getViaTmpUnchecked newkey $ \t -> do
 				-- Make a hard link to the old backend's
 				-- cached key, to avoid wasting disk space.
 				liftIO $ createLink src t