Added configurable selectionbox width. Min width = 1, Max = 5

For those who think Minetest's default selectionbox is too garish (or not garish enough).

Former default was 3, current default is 2. Max is 5, Min is 1. (Minecraft width is about 1)

Setting (minetest.conf.example):
```
# Width of the selectionbox's lines (Between 1 and 5)
#selectionbox_width = 2
```