pfctl: userspace adaptive syncookies configration

Hook up the userspace bits to configure syncookies in adaptive mode.

MFC after: 1 week
Sponsored by: Modirum MDPay
Differential Revision: https://reviews.freebsd.org/D32136

(cherry picked from commit 5062afff9de7e67da96e3f0dcb9d8bbd5a4e1c5b)