Resolvers: add secondary address for DNS forwarding
This PR adds possibility to define secondary IPs for DNS forwarding inside /etc/resolver/dns_servers config files. It can be used in pure forwarding mode or in TLS forwarding mode (DNS-over-TLS).
Fixes https://gitlab.labs.nic.cz/turris/turris-os-packages/issues/423 Fixes turris/turris-os-packages#349
Edited by Josef Schlehofer