fs: change Config callback into state based callback #3455

This is a very large change which turns the post Config function in
backends into a state based call and response system so that
alternative user interfaces can be added.

The existing config logic has been converted, but it is quite
complicated and folloup commits will likely be needed to fix it!

Follow up commits will add a command line and API based way of using
this configuration system.

backend/amazonclouddrive/amazonclouddrive.go

@@ -69,12 +69,10 @@ func init() {
 		Prefix:      "acd",
 		Description: "Amazon Drive",
 		NewFs:       NewFs,
-		Config: func(ctx context.Context, name string, m configmap.Mapper) error {
-			err := oauthutil.Config(ctx, "amazon cloud drive", name, m, acdConfig, nil)
-			if err != nil {
-				return errors.Wrap(err, "failed to configure token")
-			}
-			return nil
+		Config: func(ctx context.Context, name string, m configmap.Mapper, config fs.ConfigIn) (*fs.ConfigOut, error) {
+			return oauthutil.ConfigOut("", &oauthutil.Options{
+				OAuth2Config: acdConfig,
+			})
 		},
 		Options: append(oauthutil.SharedOptions, []fs.Option{{
 			Name:     "checkpoint",