kernel - Remove unneeded critical sections from VM code, add pmap asserts

kernel - Remove unneeded critical sections from VM code, add pmap asserts

* Various bits of VM code now only need vm_token and no longer need
  a critical section.

* Add pmap_page_assertzero() assertions in a couple of places when
  PMAP_DEBUG is enabled.

* Add PMAP_DEBUG as a global kernel config option.

* Be a bit more conservative and clear PG_ZERO for all page table
  pages and not just terminal page table pages.  When such pages
  are finally freed they will be zero again due to the way the
  pmap code works but don't make that assumption.