pf: Add counters for syncookies

Count when we send a syncookie, receive a valid syncookie or detect a
synflood.

Reviewed by: kbowling
MFC after: 1 week
Sponsored by: Modirum MDPay
Differential Revision: https://reviews.freebsd.org/D31713

(cherry picked from commit 4cab80a8dfecdf16333a1113513e046b9f4dd7f6)