MFP r291227, r291228, and r292618.

r291227:
s/is->is_conn/ic to shorten things a bit.

r291228:
Do not generate PDUs with payload greater than max_data_segment_length.

It is perhaps preferable to have a separate limit for send instead of
reusing the receive limit. I'll discuss with trasz@ and mav@ before
pulling this into head.

r292618:
Add comment to go with r291228.