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/Bup.hs

@@ -25,6 +25,7 @@ import Config.Cost
 import qualified Remote.Helper.Ssh as Ssh
 import Remote.Helper.Special
 import Remote.Helper.Messages
+import Remote.Helper.Export
 import Utility.Hash
 import Utility.UserInfo
 import Annex.UUID
@@ -61,11 +62,7 @@ gen r u c gc = do
 		, lockContent = Nothing
 		, checkPresent = checkPresentDummy
 		, checkPresentCheap = bupLocal buprepo
-		, storeExport = Nothing
-		, retrieveExport = Nothing
-		, removeExport = Nothing
-		, checkPresentExport = Nothing
-		, renameExport = Nothing
+		, exportActions = exportUnsupported
 		, whereisKey = Nothing
 		, remoteFsck = Nothing
 		, repairRepo = Nothing