selftest: packetdrill: Import sockopt-fastopen-key.pkt

selftest: packetdrill: Import sockopt-fastopen-key.pkt

sockopt-fastopen-key.pkt does not have the non-experimental
version, so the Experimental version is converted, FOEXP -> FO.

The test sets net.ipv4.tcp_fastopen_key=0-0-0-0 and instead
sets another key via setsockopt(TCP_FASTOPEN_KEY).

The first listener generates a valid cookie in response to TFO
option without cookie, and the second listner creates a TFO socket
using the valid cookie.

TCP_FASTOPEN_KEY is adjusted to use the common key in default.sh
so that we can use TFO_COOKIE and support dualstack.  Similarly,
TFO_COOKIE_ZERO for the 0-0-0-0 key is defined.

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20250927213022.1850048-13-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>