- Use _PHOLD and move it before a PROC_UNLOCK to reduce the number of

- Use _PHOLD and move it before a PROC_UNLOCK to reduce the number of
  mutex operations in kthread_create().
- Lock a kthread's proc before changing its parent via proc_reparent().
- Test P_KTHREAD not P_SYSTEM in kthread_suspend() and kthread_resume().
  P_SYSTEM just means that the process shouldn't be swapped and is used
  for vinum's daemon for example.
- Lock all the signal state used for suspending and resuming kthreads with
  the proc lock.