CHOP-960: Validate cmd_line.xml for map-to real settings.xml vars.

A small, fixed set of cmd_line.xml switches can't reasonably be mapped to
settings variables, mostly because they affect the settings machinery itself.
Other than those, every new cmd_line.xml switch should map-to a settings
variable. Validate that only the known set does not have map-to; validate that
map-to variable actually exists.