Creative: Add 'creative' privilege for survival servers

This adds a 'creative' privilege to survival servers which OPs can bestow
on admin or competent builders to give access to the creative inventory.

mods/creative/init.lua

 creative = {}
 
+minetest.register_privilege("creative", {
+	description = "Allow player to use creative inventory",
+	give_to_singleplayer = false
+})
+
 local creative_mode_cache = minetest.settings:get_bool("creative_mode")
 
 function creative.is_enabled_for(name)
-	return creative_mode_cache
+	return creative_mode_cache or
+		minetest.check_player_privs(name, {creative = true})
 end
 
 dofile(minetest.get_modpath("creative") .. "/inventory.lua")