Consistently bias pushout toward origin

Under pathological circumstances such as players
getting stuck near mapgen_limit, we don't want them getting stuck outside the bounds and then
pushed up past the ceiling.  Instead always try to
bias the push toward the origin, under the
assumption that some place near there should
be safe.

mods/nc_player_setup/step_pushout.lua

@@ -66,10 +66,14 @@ nodecore.register_playerstep({
 					return pushto(p)
 				end
 			end
+			local function bias(n)
+				return n + ((n > 0) and math_random(-6, 4)
+					or math_random(-4, 6))
+			end
 			return pushto({
-					x = pos.x + math_random(-5, 5),
-					y = pos.y + math_random(-3, 7),
-					z = pos.z + math_random(-5, 5)
+					x = bias(pos.x),
+					y = bias(pos.y),
+					z = bias(pos.z)
 				})
 		end
 	})