pkill - Fix a problem identifying system threads.

- As of commit 65a0e3d68b951f39cc39be7808e11e9de908d9fc the test to
  check whether a process is a system one or not was replaced by a
  wrong one. Restore that behaviour so that they are excluded by pkill(1).
kernel - Rename unused flag.

- This flag used to identify whether a process was a system one.
  Currently, we can use P_SYSTEM flag for that.
kernel/ndis - Fix flag usage in a macro.

- For identifying system processes, Use P_SYSTEM for
  AT_PASSIVE_LEVEL() macro.