fix a potential race condition for pre-login and ext auth

hooks

doing something like this:

err = provider.updateUser(u)
...
return provider.userExists(username)

could be racy if another update happen before

provider.userExists(username)

also pass a pointer to updateUser so if the user is modified inside
"validateUser" we can just return the modified user without do a new
query

sftpd/server.go

@@ -323,7 +323,7 @@ func (c *Configuration) configureLoginBanner(serverConfig *ssh.ServerConfig, con
 }
 
 func (c *Configuration) configureKeyboardInteractiveAuth(serverConfig *ssh.ServerConfig) {
-	if len(c.KeyboardInteractiveHook) == 0 {
+	if c.KeyboardInteractiveHook == "" {
 		return
 	}
 	if !strings.HasPrefix(c.KeyboardInteractiveHook, "http") {