send_much(force, reason, maxburst) - this function
attempts to send as many packets as the current sent window allows.
It also keeps track of how many packets it has sent, and limits to the
total to maxburst.
The function output(seqno, reason) sends one packet with the given sequence number and updates the maximum sent sequence number variable (maxseq_) to hold the given sequence number if it is the greatest sent so far. This function also assigns the various fields in the TCP header (sequence number, timestamp, reason for transmission). This function also sets a retransmission timer if one is not already pending.