_mdns_get_esp_netif() function

Gets the actual esp_netif pointer from the internal network interface list The supplied ordinal number could - point to a predef netif -> "STA", "AP", "ETH" - if no entry in the list (NULL) -> check if the system added this netif - point to a custom netif -> just return the entry in the list - users is responsible for the lifetime of this netif (to be valid between mdns-init -> deinit)