Preview tag updated - possible short term instability

[Matthew Dillon]

    I am doing some fairly significant cleanups of the kernel startup
    code and the kernel virtual memory system and it is possible that
    I may introduce bugs.  I have synchronized the preview tag so it covers
    work to date, before I start committing the cleanups.

    Amoung the cleanups occuring:

    * Make kernel_object a struct vm_object instead of a pointer

    * Make kernel_object use indexes relative to address 0 instead of
      the base of kernel memory (which removes a large chunk of arithmatic
      used to convert KVA addresses into kernel_object page indexes and
      makes everything easier to read).

    * Remove kmem_object entirely

    * Cleanup the ABI between machine-dependant code and machine-independant
      code with respect to specifying the kernel virtual address space and
      the physical address space.

    And other stuff.

						-Matt