fs: deglobalise the config #4685

This is done by making fs.Config private and attaching it to the
context instead.

The Config should be obtained with fs.GetConfig and fs.AddConfig
should be used to get a new mutable config that can be changed.

cmd/rc/rc.go

@@ -177,7 +177,7 @@ func doCall(ctx context.Context, path string, in rc.Params) (out rc.Params, err
 	}
 
 	// Do HTTP request
-	client := fshttp.NewClient(fs.Config)
+	client := fshttp.NewClient(fs.GetConfig(ctx))
 	url += path
 	data, err := json.Marshal(in)
 	if err != nil {