stand/lua: Call menu_entries if it's a function

If we've fetched menu.entries and it turns out it's a function, call it to
get the actual menu entries.

This will be used to swap multi-/single- user boot options if we're booting
single user by default (boot_single="YES" in loader.conf(5)). It can also be
used fairly easily for other non-standard situations.

stand/lua/drawer.lua

@@ -167,7 +167,9 @@ function drawer.drawmenu(m)
 	local alias_table = {};
 	local entry_num = 0;
 	local menu_entries = m.entries;
-
+	if (type(menu_entries) == "function") then
+		menu_entries = menu_entries();
+	end
 	for line_num, e in ipairs(menu_entries) do
 		-- Allow menu items to be conditionally visible by specifying
 		-- a visible function.