Add a DNS 'monitor'
After: https://leap.se/code/issues/4633 https://github.com/leapcode/bitmask_client/pull/496
We need a 'monitor' to check if our custom dns is in place or if @resolvconf@ (the user, other app, etc) has modified the @resolv.conf@ and warn the user of a possible dns leakage. We may want to give the user the option to write again our custom @resolv.conf@.
(from redmine: created on 2013-12-20, closed on 2014-07-10, relates #4633 (closed), relates #5322 (closed), relates #5738 (closed))