periodic: Make security diff(1) output as small is possible

Make, by default, security diff(1) produce a unified output with a context of
zero (0) lines. This reduces output of unrelated lines in e-mails delivered
to root.

PR: 270266
Approved by: jrm (mentor), karels
MFC after: 1 month
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D43071