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/WebDAV.hs

@@ -72,7 +72,7 @@ gen r u rc gc rs = do
 	c <- parsedRemoteConfig remote rc
 	new
 		<$> pure c
-		<*> remoteCost gc expensiveRemoteCost
+		<*> remoteCost gc c expensiveRemoteCost
 		<*> mkDavHandleVar c gc u
   where
 	new c cst hdl = Just $ specialRemote c