destiny: set ack bit for (almost) all segments after syn

The current implementation does not set the ack bit
for outgoing data segments and the fin segment.

However, RFC793 states that all segments
should have an ack bit set in order to present a valid
ack nr. in outgoing segments.

Currently, data segments and acknowledgement segments
are distinguished by the existence of their ack bit.
With the new assumption, that both of these types of
segments need an ack bit set, I had to change several
parts of the current implementation to make this
decision by inspecting the payload size.