kernel - Make pcb_onfault more robust.

kernel - Make pcb_onfault more robust.

* Record the expected kernel stack pointer along with the pcb_onfault
  action.  Adjust the trap code to only take the action if the frame's
  stack pointer matches the recorded expected stack pointer.

  Otherwise this might be a recursive trap and we definitely do NOT want
  to execute the on-fault stuff in that situation.

* Prior to these changes recursive traps during uiomove()s could result
  in a kernel stack so corrupt that finding the actual cause of the panic
  becomes impossible.  This is believed to be making life difficult for us
  trying to track down a particular i386 panic.

* On x86-64 we had to increase the size of the pcb structure.  kgdb on
  kernel cores and live kernels will be effected (needs recompile).