[$] Simpler management of the huge zero folio

One might imagine that managing a page full of zeroes would be a relativelystraightforward task; there is, after all, no data of note that must bepreserved there. The management of the huge zero folio in the kernel,though, shows that life is often not as simple as it seems. Tradeoffsbetween conflicting objectives have driven the design of this corefunctionality in different directions over the years, but much of theassociated complexity may be about to go away.