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/Helper/Export.hs

@@ -0,0 +1,21 @@
+{- exports to remotes
+ -
+ - Copyright 2017 Joey Hess <id@joeyh.name>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Remote.Helper.Export where
+
+import Annex.Common
+import Types.Remote
+
+exportUnsupported :: ExportActions Annex
+exportUnsupported = ExportActions
+	{ exportSupported = return False
+	, storeExport = \_ _ _ _ -> return False
+	, retrieveExport = \_ _ _ _ -> return (False, UnVerified)
+	, removeExport = \_ _ -> return False
+	, checkPresentExport = \_ _ -> return False
+	, renameExport = \_ _ _ -> return False
+	}