refactor ExportActions

This will allow disabling exports for remotes that are not configured to
allow them. Also, exportSupported will be useful for the external
special remote to probe.

This commit was supported by the NSF-funded DataLad project

Remote/S3.hs

@@ -39,6 +39,7 @@ import Config.Cost
 import Remote.Helper.Special
 import Remote.Helper.Http
 import Remote.Helper.Messages
+import Remote.Helper.Export
 import qualified Remote.Helper.AWS as AWS
 import Creds
 import Annex.UUID
@@ -84,11 +85,7 @@ gen r u c gc = do
 			, lockContent = Nothing
 			, checkPresent = checkPresentDummy
 			, checkPresentCheap = False
-			, storeExport = Nothing
-			, retrieveExport = Nothing
-			, removeExport = Nothing
-			, checkPresentExport = Nothing
-			, renameExport = Nothing
+			, exportActions = exportUnsupported
 			, whereisKey = Just (getWebUrls info)
 			, remoteFsck = Nothing
 			, repairRepo = Nothing