In order to allow single kernel to use PAE pagetables on i386 if hardware supports it, and fall back to classic two-level paging structures if not, superpage code should be able to adopt to either 2M or 4M superpages size. There I make MI VM structures large enough to track the biggest possible superpage, by allowing architecture to define VM_NFREEORDER_MAX and VM_LEVEL_0_ORDER_MAX constants. Corresponding VM_NFREEORDER and VM_LEVEL_0_ORDER symbols can be defined as runtime values and must be less than the _MAX constants. If architecture does not define _MAXs, it is assumed that _MAX == normal constants.

Tested by: pho (as part of the large patch)