Add tnt_knockback flag for entities

When set to true, entities will be knocked back when affected by TNT
explosions.  Also ignore '__builtin:item' entities to reduce lag, and
replace tabs with spaces in 'mcl_explosions/init.lua'.

mods/CORE/mcl_explosions/init.lua

@@ -253,16 +253,17 @@ local function trace_explode(pos, strength, raydirs, radius, drop_chance)
 
   -- Trace rays for entity damage
   for _, obj in pairs(objs) do
-    -- Object position and direction to explosion center
-    local opos = obj:get_pos()
+    local ent = obj:get_luaentity()
 
-    if obj:get_luaentity() ~= nil or obj:is_player() then
+    -- Ignore items to lower lag
+    if obj:is_player() or (ent and ent.name ~= '__builtin.item') then
+      local opos = obj:get_pos()
       local collisionbox = nil
 
       if obj:is_player() then
         collisionbox = { -0.3, 0.0, -0.3, 0.3, 1.77, 0.3 }
-      elseif obj:get_luaentity().name then
-        local def = minetest.registered_entities[obj:get_luaentity().name]
+      elseif ent.name then
+        local def = minetest.registered_entities[ent.name]
         collisionbox = def.collisionbox
       end
 
@@ -334,6 +335,8 @@ local function trace_explode(pos, strength, raydirs, radius, drop_chance)
 
         if obj:is_player() then
           obj:add_player_velocity(vector.multiply(punch_dir, impact * 20))
+        elseif ent.tnt_knockback then
+          obj:add_velocity(vector.multiply(punch_dir, impact * 20))
         end
       end
     end