Fix "cam2_turnmultiplier"

In splitscreen stuff, player 2's "cam2_turnmultiplier" implementation was wrong compared to player 1's "cam_turnmultiplier".

Worst of all, the default multiplier of 1.0 actually resulted in player 2 having absolutely no turning speed at all.

This merge request fixes that, by making player 2's "cam2_turnmultiplier" work in the same way that player 1's "cam_turnmultiplier" already works.