maintain set of files pendingAdd

Kqueue needs to remember which files failed to be added due to being open,
and retry them. This commit gets the data in place for such a retry thread.

Broke KeySource out into its own file, and added Eq and Ord instances
so it can be stored in a Set.

Types/Backend.hs

@@ -10,13 +10,7 @@
 module Types.Backend where
 
 import Types.Key
-
-{- The source used to generate a key. The location of the content
- - may be different from the filename associated with the key. -}
-data KeySource = KeySource
-	{ keyFilename :: FilePath
-	, contentLocation :: FilePath
-	}
+import Types.KeySource
 
 data BackendA a = Backend
 	{ name :: String