Merge aarp.c:1.45 from HEAD to RELENG_7:

Merge aarp.c:1.45 from HEAD to RELENG_7:

  Add additional robustness to at_aarpinput() by testing for broadcast
  addresses as the source of an AARP request.  While this PR was submitted
  in the context of work in OpenBSD to port netatalk (in 1997), I've
  synchronized the code more to our ARP input routine, which had similar
  requirements.

  Submitted by:   Denton Gentry
  PR:             kern/4184