convert fsckKey to a Maybe

This way it's clear when a backend does not implement its own fsck checks.

Backend/SHA.hs

@@ -32,7 +32,7 @@ genBackend size
 		b = Backend
 			{ name = shaName size
 			, getKey = keyValue size
-			, fsckKey = checkKeyChecksum size
+			, fsckKey = Just $ checkKeyChecksum size
 			}
 
 genBackendE :: SHASize -> Maybe Backend

Backend/URL.hs

@@ -21,7 +21,7 @@ backend :: Backend
 backend = Backend {
 	name = "URL",
 	getKey = const (return Nothing),
-	fsckKey = const (return True)
+	fsckKey = Nothing
 }
 
 fromUrl :: String -> Key

Backend/WORM.hs

@@ -18,7 +18,7 @@ backend :: Backend
 backend = Backend {
 	name = "WORM",
 	getKey = keyValue,
-	fsckKey = const (return True)
+	fsckKey = Nothing
 }
 
 {- The key includes the file size, modification time, and the