serve ftp: Use new facilities in goftp to fix and simplify auth proxy #4394

- Use Driver.CheckPasswd instead of server.CheckPasswd
- Make server.CheckPasswd return an error
- Remove awful findID to find parent function hack
- Remove Driver.Init as it is no longer called
- Fix backwards incompatible PublicIp -> PublicIP change

See: https://gitea.com/goftp/server/issues/117

cmd/serve/ftp/ftp_test.go

@@ -8,7 +8,6 @@
 package ftp
 
 import (
-	"fmt"
 	"testing"
 
 	_ "github.com/rclone/rclone/backend/local"
@@ -17,7 +16,6 @@ import (
 	"github.com/rclone/rclone/fs/config/configmap"
 	"github.com/rclone/rclone/fs/config/obscure"
 	"github.com/stretchr/testify/assert"
-	"github.com/stretchr/testify/require"
 	ftp "goftp.io/server"
 )
 
@@ -70,10 +68,3 @@ func TestFTP(t *testing.T) {
 
 	servetest.Run(t, "ftp", start)
 }
-
-func TestFindID(t *testing.T) {
-	id, err := findID([]byte("TestFindID("))
-	require.NoError(t, err)
-	// id should be the argument to this function
-	assert.Equal(t, fmt.Sprintf("%p", t), id)
-}