kernel - Implement segment pmap optimizations for x86-64 (3)

kernel - Implement segment pmap optimizations for x86-64 (3)

* Fix pmap optimization bugs triggered by XORG (startx) and postgres/mmap

* The simple-mode pmaps embedded in VM objects do not have the PML4 or PDP
  layer.  This caused pmap_scan() to miss pages, resulting in an assertion
  and panic during object frees if the objects were large enough.

* Improve postgres 9.2/mmap, still more work to go.