pfsync: Expose PFSYNCF_OK flag to userspace

Add 'syncok' field to ifconfig's pfsync interface output. This allows
userspace to figure out when pfsync has completed the initial bulk
import.

Reviewed by: donner
MFC after: 2 weeks
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D29948

(cherry picked from commit 5f5bf88949df421ec3ccd7e14a999e8d8b9251bf)