make dry stone wall an obstacle and fix its description

[NetSysFire]

Summary

Balance "Dry stone walls now need to be climbed over"

Purpose of change

I was building wattle fences and noticed that these need to be climbed over, while much bigger dry stone walls do not. This adjusts that.

Describe the solution

1. Add THIN_OBSTACLE, CLIMBABLE and BURROWABLE to the terrain and make its move cost 0, plus add the examine action and now it behaves like a proper fence. As its just a pile of arranged stone, stuff can burrow under it.
2. Fix its description. Full height dry stone walls do not exist anymore as they were never feasible.
3. Bonus fix: When I adjusted the clay pot some PRs earlier to make it an open container, I forgot to remove the lid part from its description.

Describe alternatives you've considered

Removing half-height from the description, too. But it kind of sounds right and intuitive?

Testing

Applied changes locally.

Additional context