kernel - Fix vkernel_trap

kernel - Fix vkernel_trap

* vkernel_trap restores the trapframe for the original vkernel call to
  vmspace_ctl(), but only the syscall trap code was actually setting
  up the frame for a syscall-return.

  The other calls to vkernel_trap() (signal, page-fault, other traps)
  were not properly adjusting the frame for a syscall-return and it
  is only pure luck that it didn't bite us until now.

* Add a per-platform cpu_vkernel_trap() which does the syscall-return
  fixup at the end.

Reported-by: Antonio Huete Jimenez <tuxillo@quantumachine.net>