loading root hints from a zonefile

Currently we only allow loading them from a lua table: http://knot-resolver.readthedocs.io/en/latest/modules.html#c.hints.root but some systems (like Debian) prefer to package the root hints in zonefile format.

We even have lua bindings require('zonefile'), so this should be easy to implement.

After that, we might consider to kill lib/root-hints.inc and instead load defaults from a file at start.