-initialize errno to 0 before receiving
-minimal allowed packet size is 4 (empty data packet)
-Be nice to the TFTP server - tell it that the transfer is finished
(additinal ACK at EOF, ERROR otherwise). Otherwise, it will linger
around and retransmit. This can be left out (TFTP_NOTERMINATE) if we
are really short on space.
little hacking by me. This isn't strictly identical to his 920312
release -- I've hacked it a bit -- but since we are taking over change
control it doesn't matter much.