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/Git.hs

@@ -174,7 +174,7 @@ gen r u rc gc rs
 		Nothing -> do
 			st <- mkState r u gc
 			c <- parsedRemoteConfig remote rc
-			go st c <$> remoteCost gc defcst
+			go st c <$> remoteCost gc c defcst
 		Just addr -> Remote.P2P.chainGen addr r u rc gc rs
   where
 	defcst = if repoCheap r then cheapRemoteCost else expensiveRemoteCost