MFC r263289: Update NetBSD Foundation copyrights to 2-clause BSD

  The NetBSD Foundation states "Third parties are encouraged to change the
  license on any files which have a 4-clause license contributed to the
  NetBSD Foundation to a 2-clause license."

  This change removes clauses 3 and 4 from copyright / license blocks that
  list The NetBSD Foundation as the only copyright holder.

Sponsored by:	The FreeBSD Foundation