ask "Retrieve non-secure images?" just once per page

[jidanni@xxxxxxxxxxx]

I set w3m-confirm-leaving-secure-page to nil, but what about
https://www.mrbattery.com.tw/
We get asked "You are retrieving non-secure image. Continue?(y or n)"
over and over, once for each image on the page!

Perhaps it should just ask "Retrieve non-secure images?" just once for
the whole page, and not at all if w3m-confirm-leaving-secure-page is nil.