Cockpitvoxel

looks like I misunderstood #540 a little. Cockpit voxels are not only checked for existence on building (@mrzzzrm the police still has neither a cockpit nor a crucial voxel)
Ships are also wrecked when the cockpit is completely destoyed. This makes many ships even more of a oneshot. Tell me what you think about it, I can leave out the wrecking part.