Only set retx passes (and print the count) when retx will actually happen

i_nb_retx rounds won't be required unless packets will actually be retransmitted - ie if there is a socket to push retransmits out of. Prevents wasted cycles checking if reordertp is only being used for reordering packets on the input.

Also, i_retx_fd should be checked against -1 for initialisation otherwise we incorrectly present a debug message.