This is an implementation of the client side of TCP Fast Open (TFO)
[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 names of some existing fastopen sysctls have changed (e.g.,
net.inet.tcp.fastopen.enabled -> net.inet.tcp.fastopen.server_enable).