Generally improve documentation of kern_prot.c:
Generally improve documentation of kern_prot.c:

o Add comments for:
  - kern.security.suser_permitted
  - p_cansee()
  - p_cansignal()
  - p_cansched()
  - kern.security.unprivileged_procdebug_permitted
  - p_candebug()

Update copyright.

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