security/sshguard: Add default "NO" to rcvar
Without a default rcvar set to NO, there are warnings like this:
/etc/rc: WARNING: $sshguard_enable is not set properly - see rc.conf(5)?
PR: 298781
Approved by: osa, vvd (Mentors, implicit)
MFH: 2026Q3
(cherry picked from commit 79e6862d9b008738beec5da53a29a61df69ef60d)