Page tables no longer occupy space in the processes vm_map. This
means that page tables can now be allocated from pmap_enter() when the
first entry is made in the page table rather than relying on taking
a section fault and faulting in a page table. This also lowers
the overheads in creating and deleting page tables and simplfies
cpu_fork().
A result of this change is that problems with mapping zero page and
an associated pagetable when execing from a shared vm_map disappear
fixing PR5178.
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