dns: add unbound and local-unbound

[tschettervictor]

This PR allows users to specify a resolvers variable inside /usr/local/etc/bastille/bastille.conf for auto adding jails to the specified resolver(s). For now only unbound and local-unbound are supported.

The dns.conf file should go in the main bastille config directory at /usr/local/etc/bastille/dns.conf and should contain the following contents.

The two following values are included in the config file of this PR, so make sure to diff or manually add the entries to /usr/local/etc/bastille/bastille.conf

bastille_dns_resolvers="unbound"
bastille_dns_zone="bastille" # or any name you want for the zone

You must include bastille_dns_zone for both unbound and local-unbound

It is also possible specify multiple resolver applications as in the following example:

bastille_dns_resolvers="unbound local-unbound"
bastille_dns_zone"bastille"

Each jail should get entered into the corresponding zone when started, and removed when stopped.