Add a level of indirection to the kernel PTE table. The old

Add a level of indirection to the kernel PTE table. The old
scheme allowed for 1024 PTE pages, each containing 256 PTEs.
This yielded 2GB of KVA. This is not enough to boot a kernel
on a 16GB box and in general too low for a 64-bit machine.
By adding a level of indirection we now have 1024 2nd-level
directory pages, each capable of supporting 2GB of KVA. This
brings the grand total to 2TB of KVA.