kernel - Add /dev/upmap and /dev/kpmap and sys/upmap.h (4)

* Properly remove pte entries for VM_MAPTYPE_UKSMAP mappings.  This maptype
  will have a NULL object.

* Fixes a vkernel panic, probably fixes a page leak in real kernels.
vkernel - Enhance panic message for freeing held page table page

* Adjust the 'freeing held page table page' to say 'freeing held pt page'
  and also include a bunch of pointer and other information.