libsa: re-send ACK for older data packets in tftp

In current tftp code we drop out-of-order packets; however, we should play
nice and re-send ACK for older data packets we are receiving. This will
hopefully stop server repeating those packets we already have received.
Note we do not answer duplicates from "previous" session (that is, session
with different port number), those will eventually time out.

Differential Revision: https://reviews.freebsd.org/D17087