Small mapobjectscale fixes

This fixes item throws Z momentum being adjusted twice by mapobjectscale due to usage of P_SetObjectMomZ (notably resulted in pitifully short throws in maps that used a low mapobjectscale) as well as startboost charge scaling not taking mobjscale into account properly.

This also fixes a mixed declaration warning in K_KartDrift since there wasn't much of a reason to ignore it although it's unrelated to the MR's point.