o Move the backing store in the top half of region 0 now that
region 4 is re-assigned to be part of the kernel.
o De-emphasize VM_MAX_ADDRESS. It's really not used anywhere and probably
means something different than the limit for process address space (we
have VM_MAXUSER_ADDRESS for that).
o Exclude the gateway page from VM_MAXUSER_ADDRESS (i.e. make it the same
as VM_MAX_ADDRESS).