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/Unlock.hs

@@ -41,6 +41,8 @@ perform dest key = do
 	inbackend <- Backend.hasKey key
 	when (not inbackend) $
 		error "content not present"
+	
+	checkDiskSpace key
 
 	g <- Annex.gitRepo
 	let src = gitAnnexLocation g key