MFC r303792:
Fix various bugs in relation to the I-DATA chunk support
This is joint work with rrs.

MFC r303793:
Mark an unused parameter as such.

MFC r303798:
Don't modify a structure without holding a reference count on it.

MFC r303813:
Remove stream queue entry consistently from wheel.
While there, improve the handling of drain.

MFC r303819:
Consistently check for unsent data on the stream queues.

MFC r303831:
Fix a locking issue found by stress testing with tsctp.
The inp read lock neeeds to be held when considering control->do_not_ref_stcb.

MFC r303834:
Fix the sending of FORWARD-TSN and I-FORWARD-TSN chunks. The
last SID/SSN pair wasn't filled in.
Thanks to Julian Cordes for providing a packetdrill script
triggering the issue and making me aware of the bug.

Approved by: re (kib)