MFC r316548:

Quiet 450.status-security when *_inline="YES"

Previously, 450.status-security would always set rc=3 in inline mode,
because it doesn't know whether "periodic security" is going to find
anything interesting. But this annoyingly results in daily reports that
simply say "Security check: \n\n-- End of daily output --".

This change fixes that by testing whether "periodic security" printed
anything, and setting 450.status-security's exit status to 3 if it did. An
alternative would be to change the exit status of periodic(8) to be the
worst of its scripts' exit statuses, but that would be a more intrusive
change.

Reviewed by: brian
Differential Revision: https://reviews.freebsd.org/D10267