Bring in all of Joe Talbott's SMP virtual kernel work to date, which makes

Bring in all of Joe Talbott's SMP virtual kernel work to date, which makes
virtual kernel builds with SMP almost get through a full boot.  This work
includes:

    * Creation of 'cpu' threads via libthread_xu
    * Globaldata initialization
    * AP synchronization
    * Bootstrapping to the idle thread
    * SMP pmap (mmu) functions
    * IPI handling

My part of this commit:

    * Bring all the signal interrupts under DragonFly's machine independant
      interrupt handler API.  This will properly deal with the MP lock
      and critical section handling.

    * Some additional pmap bits to handle SMP invalidation issues.

Submitted-by: Joe Talbott <josepht@cstone.net>
Additional-bits-by: Matt Dillon