LWIP_EVENT_API macro

LWIP_EVENT_API and LWIP_CALLBACK_API: Only one of these should be set to 1. LWIP_EVENT_API==1: The user defines lwip_tcp_event() to receive all events (accept, sent, etc) that happen in the system. LWIP_CALLBACK_API==1: The PCB callback function is called directly for the event. This is the default.