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.

Remote/Glacier.hs

@@ -89,8 +89,9 @@ glacierSetup' ss u mcreds c gc = do
 	(c', encsetup) <- encryptionSetup c gc
 	c'' <- setRemoteCredPair encsetup c' gc (AWS.creds u) mcreds
 	let fullconfig = c'' `M.union` defaults
-	when (ss == Init) $
-		genVault fullconfig gc u
+	case ss of
+		Init -> genVault fullconfig gc u
+		_ -> return ()
 	gitConfigSpecialRemote u fullconfig "glacier" "true"
 	return (fullconfig, u)
   where