kernel - Rewrite the x86-64 pmap code

kernel - Rewrite the x86-64 pmap code

* Use unassociated VM pages (without a VM object) for all page table pages.

* Remove kptobj and pmap->pm_pteobj.

* For the moment implement a Red-Black tree for pv_entry_t manipulation.
  Revamp the pindex to include all page table page levels, from terminal
  pages to the PML4 page.  The hierarchy is now arranged via the PV system.

* As before, the kernel page tables only use PV entries for terminal pages.

* Refactor the locking to allow blocking operations during deep scans.
  Individual PV entries are now locked and critical PMAP operations do not
  require the pmap->pm_token.  This should greatly improve threaded
  program performance.

* Fix kgdb on the live kernel (pmap_extract() was not handling short-cutted
  page directory pages).