Implement nearly all the remaining items required to allow the virtual kernel

Implement nearly all the remaining items required to allow the virtual kernel
to actually execute code on behalf of a virtualized user process.  The
virtual kernel is now able to execute the init binary through to the point
where it sets up a TLS segment.

* Create a pseudo tf_trapno called T_SYSCALL80 to indicate system call traps.

* Add MD shims when creating or destroying a struct vmspace, allowing the
  virtual kernel to create and destroy real-kernel vmspaces along with.

  Add appropriate calls to vmspace_mmap() and vmspace_mcontrol() to map
  memory inside the user process vmspace.  The memory is mapped VPAGETABLE
  and the page table directory is set to point to the pmap page directory.

* Clean up user_trap, handle T_PAGEFLT properly.

* Implement go_user().  It calls vmspace_ctl(... VMSPACE_CTL_RUN) and
  user_trap() in a loop, allowing the virtual kernel to 'run' a user
  mode context under its control.

* Reduce VM_MAX_USER_ADDRESS to 0xb8000000 for now, until I figure out the
  best way to have the virtual kernel query the actual max user address from
  the real kernel.

* Correct a pm_pdirpte assignment.  We can't look up the PTE until after
  we have entered it into the kernel pmap.