Squashed commit of the following:

commit dea04ad13760ff21c2e6f71a8b35bf2ef2b5b761
Author: Gísli Kristjánsson <gislik@hamstur.is>
Date:   Thu Jan 23 23:17:16 2014 +0000

    Support for the $MINTTL environment variable in wich dnscache
    caches and returns the minimum of the actual ttl and $MINTTL