Dnsdist and auth to use the same config file selection logic as recursor

[LeePorte]

• Program: dnsdist, auth
• Issue type: Feature request

Short description

I'd like dnsdist and auth to adopt the same logic for the default configuration file as is in place for recursor, so that a yml config file takes precedence over a conf config file

Usecase

The use case is that if dnsdist or auth has a yml config file present in this systemdir then it uses the yml file over the conf file. This then gives the same user experience as is present in recursor and reduces friction in adopting using YAML for configuration as the Lua files are being phased out

Description

When a dnsdist.yml file is present in the dnsdist sysdir use the YAML configuration over the Lua configuration automatically.
When a pdns.yml file is present in the auth sysdir use the YAML configuration over the Lua configuration automatically.

[rgacogne]

We carefully decided not to that in 2.0.0 because DNSdist Defender already comes with .yml configuration files so there was a potential for nasty surprises, but this is likely to happen in 2.1.0.