added GETWANTED, SETWANTED for Tobias's flickr remote

This was unexpectedly difficult because of a depdenency cycle. To parse a
preferred content expression involves several things that need to operate
on the list of remotes. Which needs Remote.External. The only way to avoid
this cycle (I tried breaking it at several points) was to skip parsing the
expression in SETWANTED.

That's sorta ok, because git-annex already has to deal with unparsable
preferred content expressions being stored, in order to handle eg,
upgrades. But I'm still not very happy that I cannot check it.

I feel this is a strong indication that I need to beware of further
bloating the special remote protocol interface.

Remote/External/Types.hs

@@ -33,6 +33,7 @@ module Remote.External.Types (
 import Common.Annex
 import Annex.Exception
 import Types.Key (file2key, key2file)
+import Types.StandardGroups (PreferredContentExpression)
 import Utility.Metered (BytesProcessed(..))
 import Logs.Transfer (Direction(..))
 import Config.Cost (Cost)
@@ -167,6 +168,8 @@ data RemoteRequest
 	| SETCREDS Setting String String
 	| GETCREDS Setting
 	| GETUUID
+	| SETWANTED PreferredContentExpression
+	| GETWANTED
 	deriving (Show)
 
 instance Receivable RemoteRequest where
@@ -178,6 +181,8 @@ instance Receivable RemoteRequest where
 	parseCommand "SETCREDS" = parse3 SETCREDS
 	parseCommand "GETCREDS" = parse1 GETCREDS
 	parseCommand "GETUUID" = parse0 GETUUID
+	parseCommand "SETWANTED" = parse1 SETWANTED
+	parseCommand "GETWANTED" = parse0 GETWANTED
 	parseCommand _ = parseFail
 
 -- Responses to RemoteRequest.