tcp: Implement RFC4653 Non-Congestion Robustness (NCR)

tcp: Implement RFC4653 Non-Congestion Robustness (NCR)

It is enabled by default and can be disabled using sysctl node:
net.inet.tcp.ncr

As far as I have tested on heavily reordered network path, this
algorithm does avoid most of the spurious fast retransmits.  While
on the normal network path, the fast retransmits stil could be
triggered properly.