Do not clear robust lists pointers on fork. The forked child thread
lists must be functional.

Reported by: Daniel Engberg <daniel.engberg.lists@pyret.net>,
Guy Yur <guyyur@gmail.com>
Tested by: Guy Yur <guyyur@gmail.com>
Sponsored by: The FreeBSD Foundation
Approved by: re (gjb), including the KBI change