Make fire spread direction truly random

This patch initializes the random number generator used in mcl_fire with
the current Unix timestamp. It also corrects two biases in fire spread
that were caused by nodes being iterated over in a predictable way.

mods/ITEMS/mcl_fire/init.lua

@@ -36,12 +36,14 @@ local adjacents = {
 	{ x = 0, y = 0, z = 1 },
 }
 
-local function shuffle_adjacents()
-	for i = #adjacents, 1, -1 do
+math.randomseed(os.time())
+local function shuffle_table(t)
+	for i = #t, 1, -1 do
 		local r = math.random(i)
-		adjacents[i], adjacents[r] = adjacents[r], adjacents[i]
+		t[i], t[r] = t[r], t[i]
 	end
 end
+shuffle_table(adjacents)
 
 local function has_flammable(pos)
 	for k,v in pairs(adjacents) do
@@ -368,6 +370,7 @@ minetest.register_abm({
 
 local function check_aircube(p1,p2)
 	local nds=minetest.find_nodes_in_area(p1,p2,{"air"})
+	shuffle_table(nds)
 	for k,v in pairs(nds) do
 		if has_flammable(v) then return v end
 	end
@@ -409,7 +412,7 @@ else -- Fire enabled
 			local p = get_ignitable(pos)
 			if p then
 				spawn_fire(p)
-				shuffle_adjacents()
+				shuffle_table(adjacents)
 			end
 		end
 	})