Tunnels: Completely disable generation when 'cave width' >= 10.0 (#9093)

Tunnels: Completely disable generation when 'cave width' >= 10.0 (#9093)

Previously, the only way to disable the 3D noise tunnels was to set
'cave width' > 1.0, however doing so did not disable the very intensive
noise calculations or the generation loop.
All the other types of cave generation (randomwalk caves, caverns)
can already be independently and completely disabled.
This feature is now needed more because the small randomwalk caves are
now available for use as an alternative to the 3D noise tunnels.