Various improvements and bugfixes

Clear driver on leave using the detach callback (0.5.x and later)
Fix upright_sprite attachment position
Fallback to the regular node sound when metal does not exist
Set animation to "stand" on attach

init.lua

@@ -25,7 +25,7 @@ if not default.player_attached then
 	default.player_attached = {}
 end
 
-minetest.after(0.5, function()
+minetest.after(0, function()
 	boost_cart.old_player_model = not minetest.global_exists("player_api")
 end)