MFC r307901,r308180:

FreeBSD tcp stack used to inform respective congestion control module about the
loss event but not use or obay the recommendations i.e. values set by it in some
cases.

Here is an attempt to solve that confusion by following relevant RFCs/drafts.
Stack only sets congestion window/slow start threshold values when there is no
CC module availalbe to take that action. All CC modules are inspected and
updated when needed to take appropriate action on loss.

tcp_stacks/fastpath module has been updated to adapt these changes.

Note: Probably, the most significant change would be to not bring congestion
window down to 1MSS on a loss signaled by 3-duplicate acks and letting
respective CC decide that value.