Move all DNS proxy related rules into single nftables table.

[Gerrit91]

Have no idea if this is gonna work, but it would bring the following improvements:

• Only configured DNS server considered by proxy, no public servers used anymore if desired
• Potentially more than one DNS server can be configured for DNS proxy
• Fixes alternative DNS port setting not working in current implementation
• Less responsibility of metal-networker for firewall-controller features

Still missing in this refactoring:

• IPv6 compatibility
• Filter on snat rules (?)