This option was added in commit 0a100a6f1ee5 but was never completed.
In particular, there is no logic to map flowids to different listening
sockets, so it accomplishes basically the same thing as SO_REUSEPORT.
Meanwhile, we've since added SO_REUSEPORT_LB, which at least tries to
balance among listening sockets using a hash of the 4-tuple and some
optional NUMA policy.

The option was never documented or completed, and an exp-run revealed
nothing using it (or librss) in the ports tree. Moreover, it
complicates the already very complicated in_pcbbind_setup(), and the
checking in in_pcbbind_check_bindmulti() is insufficient. So, let's
remove it.

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Sponsored by: Modirum MDPay