added an optional cost= configuration to all special remotes

Note that when this is specified and an older git-annex is used to
enableremote such a special remote, it will simply ignore the cost= field
and use whatever the default cost is.

In passing, fixed adb to support the remote.name.cost and
remote.name.cost-command configs.

Sponsored-by: Dartmouth College's DANDI project

Remote/GitLFS.hs

@@ -93,7 +93,7 @@ gen r u rc gc rs = do
 		else pure r
 	sem <- liftIO $ MSemN.new 1
 	h <- liftIO $ newTVarIO $ LFSHandle Nothing Nothing sem r' gc
-	cst <- remoteCost gc expensiveRemoteCost
+	cst <- remoteCost gc c expensiveRemoteCost
 	let specialcfg = (specialRemoteCfg c)
 		-- chunking would not improve git-lfs
 		{ chunkConfig = NoChunks