Import a clean-room implementation of the experimental H-TCP congestion control

Import a clean-room implementation of the experimental H-TCP congestion control
algorithm based on the Internet-Draft "draft-leith-tcp-htcp-06.txt". It is
implemented as a kernel module compatible with the recently committed modular
congestion control framework.

H-TCP was designed to provide increased throughput in fast and long-distance
networks. It attempts to maintain fairness when competing with legacy NewReno
TCP in lower speed scenarios where NewReno is able to operate adequately. The
paper "H-TCP: A framework for congestion control in high-speed and long-distance
networks" provides additional detail.

In collaboration with:	David Hayes <dahayes at swin edu au> and
			Grenville Armitage <garmitage at swin edu au>
Sponsored by:	FreeBSD Foundation
Reviewed by:	rpaulo (older patch from a few weeks ago)
MFC after:	3 months