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/BitTorrent.hs

@@ -26,6 +26,7 @@ import Backend.URL
 import Annex.Perms
 import Annex.UUID
 import qualified Annex.Url as Url
+import Remote.Helper.Export
 
 import Network.URI
 
@@ -61,11 +62,7 @@ gen r _ c gc =
 		, lockContent = Nothing
 		, checkPresent = checkKey
 		, checkPresentCheap = False
-		, storeExport = Nothing
-		, retrieveExport = Nothing
-		, removeExport = Nothing
-		, checkPresentExport = Nothing
-		, renameExport = Nothing
+		, exportActions = exportUnsupported
 		, whereisKey = Nothing
 		, remoteFsck = Nothing
 		, repairRepo = Nothing