Timing enhancements to the FreeBSD kernel to support delay and rate based TCP mechanisms

The enhanced Round Trip Time module (h_ertt) is used by delay and rate based TCP congestion control algorithms in the Modular Congestion Control Framework for FreeBSD. Three key protocol issues need to be addressed when making robust accurate RTT estimates in the FreeBSD TCP stack: delayed acknowledgements, TCP Segmentation Offload, and Selective Acknowledgements. The h_ertt module algorithm addresses these providing an accurate robust measure of RTT.