When tunnels entirely above ground were avoided, the
missing pseudorandom calls changed the allowed caves.
Now, above ground tunnels are not placed while
still running all previous pseudorandom calls.

Mgv5/mgv7: Remove 'should make cave hole' feature
Remove ravine code

Conf.example: Update mgv5 mapgen params
Mgv7: Lava caves only below -256

Add large pseudorandom caves below -256
De-link terrain level from water_level
Cavegen: Mgv5 large cave code

Cavegen: Prevent caves from occuring above ground level, and superfluous mixing of lava and water in caves