Offset KernelPTD and KernelPTA so we can directly translate a kernel virtual

Offset KernelPTD and KernelPTA so we can directly translate a kernel virtual
address without subtracting KvaStart, simplifying the pmap code.  Also
clean up the page table initialization (make it actually work properly).

Make sure the set_sysinit_set and set_sysuninit_set sections are R/W,
because the kernel SYSINIT modifies them.

Successfully boot the virtual kernel through to the first vm_fork.