Allow vertical mouse sensitivity to be zero

This is the first issue I open on this project, so first off let me thank you for your work on EDuke32 all these years. It's my preferred port to play Duke Nukem 3D by far.

I like to play the game using a mix of classic and modern game controls, which means I like to use the keyboard to strafe and run and the mouse to look around horizontally. However, I generally dislike vertical mouse movement to interfere with player movement or aiming by default. My solution so far has been to configure vertical mouse sensitivity to a minimum or changing the config file manually to set it to zero. Allowing it to be set to zero from the game menu would be simpler and better for my use case.

MR incoming.