Add entity damage and player knockback

Could not find a way to add knockback to entities.  After adding a
velocity to them they will just slide around indefinitely.  Because of
this, knockback is only enabled on players for now.

mods/ITEMS/mcl_tnt/init.lua

@@ -103,6 +103,11 @@ minetest.register_node("mcl_tnt:tnt", {
 	_doc_items_usagehelp = S("Place the TNT and ignite it with one of the methods above. Quickly get in safe distance. The TNT will start to be affected by gravity and explodes in 4 seconds."),
 	groups = { dig_immediate = 3, tnt = 1, enderman_takable=1 },
 	mesecons = tnt_mesecons,
+	on_blast = function(pos)
+		minetest.chat_send_all("on_blast " .. minetest.pos_to_string(pos))
+	        spawn_tnt(pos, "mcl_tnt:tnt")
+		return true
+	end,
 	_on_ignite = function(player, pointed_thing)
 		tnt.ignite(pointed_thing.under)
 		return true