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/SetKey.hs

@@ -32,7 +32,8 @@ perform :: FilePath -> CommandPerform
 perform file = do
 	key <- cmdlineKey
 	-- the file might be on a different filesystem, so mv is used
-	-- rather than simply calling moveToObjectDir
+	-- rather than simply calling moveToObjectDir; disk space is also
+	-- checked this way.
 	ok <- getViaTmp key $ \dest -> do
 		if dest /= file
 			then liftIO $