update unicode FilePath handling

Based on http://hackage.haskell.org/trac/ghc/ticket/3307 ,
whether FilePath contains decoded unicode varies by OS.
So, add a configure check for it.

Also, renamed showFile to filePathToString

Backend/WORM.hs

@@ -67,5 +67,5 @@ checkKeySize key = do
 				then return True
 				else do
 					dest <- moveBad key
-					warning $ "Bad file size; moved to " ++ showFile dest
+					warning $ "Bad file size; moved to " ++ filePathToString dest
 					return False