Example usage:

    sub vcl_fetch {
	/* Spread TTLs -10%..+5% because CMS timestamps whole minutes */
	set beresp.ttl = beresp.ttl * random(0.90, 1.05);
    }

This usage was one of the very first VCL examples we ever wrote on
a whiteboard, we just never got a round tuit until now.

Fixes: #793



git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@5421 d4fa192b-c00b-0410-8231-f00ffab90ce4