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/BitTorrent.hs

@@ -60,8 +60,8 @@ list _autoinit = do
 
 gen :: Git.Repo -> UUID -> RemoteConfig -> RemoteGitConfig -> RemoteStateHandle -> Annex (Maybe Remote)
 gen r _ rc gc rs = do
-	cst <- remoteCost gc expensiveRemoteCost
 	c <- parsedRemoteConfig remote rc
+	cst <- remoteCost gc c expensiveRemoteCost
 	return $ Just Remote
 		{ uuid = bitTorrentUUID
 		, cost = cst