Add a Securelevel sub-menu to the Security configuration menu,
permitting the administrator to select a securelevel top operate
at. Include a helpfile summarizing some of the information from
init(8). This allows for explicit configuration of securelevels,
which was previously implicit in Security Profile selection.
Currently, there are no checkboxes for the active securelevel,
because sysinstall's facilities for deriving "current settings"
from rc.conf may use only one variable, not two, and I opted for
the simplest approach at this point.

Approved by: re (scottl)