A TCP connection made by sendto() with TCP_FASTOPEN set always sets the
TCP_FASTOPEN readback getsockopt. It would be (possibly more) useful to the
client application to know if a supposedly valid TFO cookie was sent on the SYN
(as opposed to the current possibilty of only having sent a TFO request) and/or
whether the server agreed that the cookie was acceptable.
TCP_INFO TCPOPT_FAST_OPEN appears to match getsockopt/TCP_FASTOPEN, for a client.
I'm not sure that we really want to go so far as knowing whether client
data-on-SYN was actually accepted with the SYN, but it's another item in the