tcp_do_output_nagle macro

This is the Nagle algorithm: try to combine user data to send as few TCP segments as possible. Only send if - no previously transmitted data on the connection remains unacknowledged or - the TF_NODELAY flag is set (nagle algorithm turned off for this pcb) or - the only unsent segment is at least pcb->mss bytes long (or there is more than one unsent segment - with lwIP, this can happen although unsent->len < mss) - or if we are in fast-retransmit (TF_INFR)