Add a debug CLI command to seed random(3).

This is a lot less useful than it could have been, as the Open Group
only mandates that:

        Like rand(), random() shall produce by default a sequence
        of numbers that can be duplicated by calling srandom() with
        1 as the seed.

But crucially leaves out *which* sequence of numbers.



git-svn-id: http://www.varnish-cache.org/svn/branches/2.0@3532 d4fa192b-c00b-0410-8231-f00ffab90ce4