ddb.onpanic to 1, change it back to 0 in sysctl.conf and make sure
postinstall installs this setting.
This avoids us trying to dump while booting from install CD, but keeps
the default the same once we are far enough through /etc/rc.d. Failing
earlier is unlikely to be recovered by an automatic reboot.
OK: core.
This replaces the previous /etc/rc.d/sysctl. Also, the variables are now
set earlier, between ipf/ipnat and the network coming up.
The rc.conf defcorename and securelevel variables are no more. You can
set them directly in sysctl.conf now.