Add option for non-jumpable fences and walls

Add an option, default disabled, to extend the collision boxes upwards.

minetest.conf.example

@@ -60,6 +60,9 @@ default:torch 99,default:cobble 99
 # Default value is true.
 #enable_bed_night_skip = true
 
+# If enabled, fences and walls cannot be jumped over.
+#enable_fence_tall = false
+
 # Whether the engine's spawn search, which does not check for a suitable
 # starting biome, is used.
 # Default value is false.