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.

Utility/DiskFree.hs

@@ -0,0 +1,32 @@
+{- disk free space checking 
+ -
+ - Copyright 2012 Joey Hess <joey@kitenet.net>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+{-# LANGUAGE ForeignFunctionInterface #-}
+
+module Utility.DiskFree ( getDiskFree ) where
+
+import Common
+
+import Foreign.C.Types
+import Foreign.C.String
+import Foreign.C.Error
+
+foreign import ccall unsafe "diskfree.h diskfree" c_diskfree
+	:: CString -> IO CULLong
+
+getDiskFree :: String -> IO (Maybe Integer)
+getDiskFree path = withFilePath path $ \c_path -> do
+	free <- c_diskfree c_path
+	ifM (safeErrno <$> getErrno)
+		( return $ Just $ toInteger free
+		, do
+			Errno i <- getErrno
+			print i
+			return Nothing
+		)
+	where
+		safeErrno (Errno v) = v == 0