[$] Simpler management of the huge zero folio

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