wrong interpretation of how part of TCP works.

Being an ACK and being a data packet are *not* mutually exclusive,
though the old code assumed that they were.

There are basically four things we might need to do:
If outgoing
    (a) Handle an outgoing ACK
    (b) Handle an outgoing data packet
If incoming
    (c) Handle an incoming ACK
    (d) Handle an incoming data packet

Note that a and b can be done on the same packet, as well as c and d. Right
now, however, our code only handles b and c. We will need to support a and
d before we can model real applications, I think!

Also, make handleTcp() more robust.