rc.d: unbreak sysctl lastload

/etc/rc.d/securelevel is supposed to run /etc/rc.d/sysctl lastload
late at boot time to apply /etc/sysctl.conf settings that fail
to apply early. However, this does not work in default configuration
because of kern_securelevel_enable="NO" by default.

Add new script /etc/rc.d/sysctl_lastload that starts unconditionally.

Reported by: Marek Zarychta
MFC after: 1 month

(cherry picked from commit f4b38c360e63a6e66245efedbd6c070f9c0aee55)