This is primarily an implementation of the client side of RFC7413. It
also includes a pre-shared key mode of operation in which the server
requires the client to be in possession of a shared secret in order to
successfully open TFO connections with that server.
The TFO code is now unconditionally compiled into the kernel and the
TCP_RFC7413 option has been removed. The TCP_RFC7413_MAX_KEYS
option has been renamed to TCP_FASTOPEN_MAX_KEYS.
A bug in tcp_fastopen_check_cookie() where V_tcp_fastopen_numkeys was
being evaluated outside of the lock has been fixed.
The names of some existing fastopen sysctls have changed (e.g.,