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/S3.hs

@@ -129,8 +129,9 @@ s3Setup' ss u mcreds c gc
 		(c', encsetup) <- encryptionSetup c gc
 		c'' <- setRemoteCredPair encsetup c' gc (AWS.creds u) mcreds
 		let fullconfig = c'' `M.union` defaults
-		when (ss == Init) $
-			genBucket fullconfig gc u
+		case ss of
+			Init -> genBucket fullconfig gc u
+			_ -> return ()
 		use fullconfig
 
 	archiveorg = do