This is 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 names of some existing fastopen sysctls have changed (e.g.,
This revision incorporates the comments made on the now-abandoned D13718.
This revision depends on D14046.