add support for readonly remotes

Currently only the web special remote is readonly, but it'd be possible to
also have readonly drives, or other remotes. These are handled in the
assistant by only downloading from them, and never trying to upload to
them.

Assistant/TransferQueue.hs

@@ -23,6 +23,7 @@ import Assistant.DaemonStatus
 import Logs.Transfer
 import Types.Remote
 import qualified Remote
+import qualified Types.Remote as Remote
 
 import Control.Concurrent.STM
 import qualified Data.Map as M
@@ -78,7 +79,7 @@ queueTransfers schedule q dstatus k f direction = do
 			-- can be uploaded to, in order to ensure all
 			-- remotes can access the content. Currently,
 			-- send to every remote we can.
-			| otherwise = return rs
+			| otherwise = return $ filter (not . Remote.readonly) rs
 		gentransfer r = Transfer
 			{ transferDirection = direction
 			, transferKey = k