Add rc.conf option to set kernel security level and modify "/etc/rc" to

Add rc.conf option to set kernel security level and modify "/etc/rc" to
use this.

Requested by: max, andreas

Note: This change just provides a convenient way to exercise existing
functionality.  Whether `kern.securelevel' is effective in increasing
system security is another issue, and one that has been well thrashed
out in the lists.