Better document security_show_{success,info,badconfig} in /etc/periodic.conf

Better document security_show_{success,info,badconfig} in /etc/periodic.conf

periodic(8) already handles the security_show_{success,info,badconfig}
variables correctly. However, those variables aren't explicitly set in
/etc/defaults/periodic.conf or anywhere else, which suggests to the user
that they shouldn't be used.

etc/defaults/periodic.conf
	Explicitly set defaults for security_show_{success,info,badconfig}

usr.sbin/periodic/periodic.sh
	Update usage string

usr.sbin/periodic/periodic.8
	Minor man page updates

One thing I'm _not_ doing is recommending setting security_output to
/var/log/security.log or adding that file to /etc/newsyslog.conf, because
periodic(8) would create it with default permissions, usually 644, and
that's probably a bad idea.

Reviewed by:	brd
MFC after:	4 weeks
Sponsored by:	Spectra Logic Corp
Differential Revision:	https://reviews.freebsd.org/D6477