KillBelowPercent-Type Flag for TechnoTypes

Add a tag like ProtectedDriver.KillBelowPercent (or simply supersede ProtectedDriver as this can replace its functionality) for TechnoTypes.

The purpose of this is to allow certain units to have more resistance to KillDriver weapons than others. If a Warhead with KillDriver=yes and KillDriver.KillBelowPercent defined is applied to a TechnoType with this new tag added, it will use the lower number of the two for the logic..