Rewrote free disk space checking code

Moving the portability handling into a small C library cleans up things
a lot, avoiding the pain of unpacking structs from inside haskell code.

Config.hs

@@ -12,7 +12,6 @@ import qualified Git
 import qualified Git.Config
 import qualified Git.Command
 import qualified Annex
-import qualified Build.SysConfig
 import Utility.DataUnits
 
 type ConfigKey = String
@@ -92,19 +91,8 @@ getTrustLevel :: Git.Repo -> Annex (Maybe String)
 getTrustLevel r = fromRepo $ Git.Config.getMaybe $ remoteConfig r "trustlevel"
 
 {- Gets annex.diskreserve setting. -}
-getDiskReserve :: Bool -> Annex Integer
-getDiskReserve sanitycheck = do
-	r <- getConfig "diskreserve" ""
-	when sanitycheck $ check r
-	return $ fromMaybe megabyte $ readSize dataUnits r
+getDiskReserve :: Annex Integer
+getDiskReserve = fromMaybe megabyte . readSize dataUnits
+	<$> getConfig "diskreserve" ""
 	where
 		megabyte = 1000000
-		check r
-			| not (null r) && not cancheck = do
-				unlessM (Annex.getState Annex.force) $
-					error $ "You have configured a diskreserve of "
-						++ r ++
-						" but disk space checking is not working"
-				return ()
-			| otherwise = return ()
-		cancheck = Build.SysConfig.statfs_sanity_checked == Just True