New struct player2_t, port centermessage to player2_t.
Look up/down works.
Mouse look works.
Jumping works.
Fix sky stetching when free look or mouse look is enabled.
Merge branch 'crispy-doom' into player2
Make jumping optional.
Retain demo compatibility, phew!
Generally suppress in-air movement when jumping is disabled.
Improve mouselook:

* Add a mousekey to temporarily enable mouselook
* Add separate mouse settings for vertical movement
* Apply novert only for translational movement
Add crosshair/laserpointer
Refine mouse look, suppress crosshair with secret message.
Fix look centering.
Show additional settings in crispy-setup
Merge branch 'player2' into crispy-doom