Introduce globals: KvaStart, KvaEnd, and KvaSize. Used by the kernel

Introduce globals: KvaStart, KvaEnd, and KvaSize.  Used by the kernel
instead of the nutty VADDR and VM_*_KERNEL_ADDRESS macros.  Move extern
declarations for these variables as well as for virtual_start, virtual_end,
and phys_avail[] from MD headers to MI headers.

Make kernel_object a global structure instead of a pointer.

Remove kmem_object and all related code (none of it is used any more).