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/Helper/Encryptable.hs

@@ -15,6 +15,7 @@ module Remote.Helper.Encryptable (
 	embedCreds,
 	cipherKey,
 	extractCipher,
+	isEncrypted,
 	describeEncryption,
 ) where
 
@@ -57,7 +58,7 @@ encryptionSetup c gc = do
 	encryption = M.lookup "encryption" c
 	-- Generate a new cipher, depending on the chosen encryption scheme
 	genCipher cmd = case encryption of
-		_ | M.member "cipher" c || M.member "cipherkeys" c || M.member "pubkeys" c -> cannotchange
+		_ | hasEncryptionConfig c -> cannotchange
 		Just "none" -> return (c, NoEncryption)
 		Just "shared" -> encsetup $ genSharedCipher cmd
 		-- hybrid encryption is the default when a keyid is
@@ -167,6 +168,15 @@ extractCipher c = case (M.lookup "cipher" c,
   where
 	readkeys = KeyIds . splitc ','
 
+isEncrypted :: RemoteConfig -> Bool
+isEncrypted c = case M.lookup "encryption" c of
+	Just "none" -> False
+	Just _ -> True
+	Nothing -> hasEncryptionConfig c
+
+hasEncryptionConfig :: RemoteConfig -> Bool
+hasEncryptionConfig c = M.member "cipher" c || M.member "cipherkeys" c || M.member "pubkeys" c
+
 describeEncryption :: RemoteConfig -> String
 describeEncryption c = case extractCipher c of
 	Nothing -> "none"