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/P2P.hs

@@ -48,7 +48,7 @@ chainGen :: P2PAddress -> Git.Repo -> UUID -> RemoteConfig -> RemoteGitConfig ->
 chainGen addr r u rc gc rs = do
 	c <- parsedRemoteConfig remote rc
 	connpool <- mkConnectionPool
-	cst <- remoteCost gc veryExpensiveRemoteCost
+	cst <- remoteCost gc c veryExpensiveRemoteCost
 	let protorunner = runProto u addr connpool
 	let withconn = withConnection u addr connpool
 	let this = Remote