libcommon: Separate body/head yaw controls

The head yaw only controls look direction, while the body yaw
controls an offset added to the gun/body direction. Both are
separate from the main delta-based "turn" control.

The head tracker device is now getting simulated yaw events.

To test this out:
- bindcontrol yawhead head-yaw
- bindcontrol yawbody head-yaw