pf: syncookie ioctl interface

Kernel side implementation to allow switching between on and off modes,
and allow this configuration to be retrieved.

MFC after: 1 week
Sponsored by: Modirum MDPay
Differential Revision: https://reviews.freebsd.org/D31139

(cherry picked from commit 231e83d3422ff58fe94de8375a9532a1726056ed)