This is an implementation of the client side of TCP Fast Open (TFO)

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).

Reviewed by:	tuexen
MFC after:	1 month
Sponsored by:	Limelight Networks
Differential Revision:	https://reviews.freebsd.org/D14047