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/Bup.hs

@@ -65,7 +65,7 @@ gen :: Git.Repo -> UUID -> RemoteConfig -> RemoteGitConfig -> RemoteStateHandle
 gen r u rc gc rs = do
 	c <- parsedRemoteConfig remote rc
 	bupr <- liftIO $ bup2GitRemote buprepo
-	cst <- remoteCost gc $
+	cst <- remoteCost gc c $
 		if bupLocal buprepo
 			then nearlyCheapRemoteCost
 			else expensiveRemoteCost