prevent exporttree=yes on remotes that don't support exports

Don't allow "exporttree=yes" to be set when the special remote
does not support exports. That would be confusing since the user would
set up a special remote for exports, but `git annex export` to it would
later fail.

This commit was supported by the NSF-funded DataLad project.

Logs/Trust.hs

@@ -67,7 +67,7 @@ trustMapLoad = do
 	overrides <- Annex.getState Annex.forcetrust
 	l <- remoteList
 	-- Exports are never trusted, since they are not key/value stores.
-	exports <- filterM (Types.Remote.exportSupported . Types.Remote.exportActions) l
+	exports <- filterM Types.Remote.isExportSupported l
 	let exportoverrides = M.fromList $
 		map (\r -> (Types.Remote.uuid r, UnTrusted)) exports
 	logged <- trustMapRaw