r300792:
  exec: Add credential change information into imgp for process_exec hook.
r300851:
  exec: get rid of one vnode lock/unlock pair in do_execve
r301580:
  Old process credentials for setuid execve must not be dereferenced when the
  process credentials were not changed.  This can happen if an error occured
  trying to activate the setuid binary.  And on error, if new credentials
  were not yet assigned, they must be freed to not create the leak.