implement exporttree=yes configuration

* Only export to remotes that were initialized to support it.
* Prevent storing key/value on export remotes.
* Prevent enabling exporttree=yes and encryption in the same remote.

SetupStage Enable was changed to take the old RemoteConfig.
This allowed only setting exporttree when initially setting up a
remote, and not configuring it later after stuff might already be stored
in the remote.

Went with =yes rather than =true for consistency with other parts of
git-annex. Changed docs accordingly.

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

Types/Remote.hs

@@ -44,8 +44,7 @@ type RemoteConfigKey = String
 
 type RemoteConfig = M.Map RemoteConfigKey String
 
-data SetupStage = Init | Enable
-	deriving (Eq)
+data SetupStage = Init | Enable RemoteConfig
 
 {- There are different types of remotes. -}
 data RemoteTypeA a = RemoteType {