add Remote.readonly

Does nothing yet.

Considered making bup readonly, but while the content can't be removed,
it is able to delete a branch, so didn't.

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

Remote/Rsync.hs

@@ -95,6 +95,7 @@ gen r u c gc = do
 				then Just $ rsyncUrl o
 				else Nothing
 			, readonly = False
+			, appendonly = False
 			, availability = if islocal then LocallyAvailable else GloballyAvailable
 			, remotetype = remote
 			, mkUnavailable = return Nothing