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