under-documented zokumbsp behaviors

This nulls out sides, seems to be a CLI flag -- doesn't look to be destructive?
```cpp
// Ignore line if both sides point to the same sector & neither side has any
// visible texture

// This fix allows people to create invisible floors by putting lower
// textures on those linedefs.
```

```cpp
// do not add segs that have the BSPNOSEG texture as lower
```