Add minetest.copy_table(table)

To get rid off the "table references".
This can be simplified to:
```Lua
ntable[k] = seen[v] or core.copy_table(v, seen)
```
Which is just as efficient.  Also, I think `copy` is a better name than `ntable` and `otable` can be simply `t`, `o`, or `x`.
Oh, and `{ }` should be `{}`, see http://dev.minetest.net/Lua_code_style_guidelines.