Add configuration test feature with exit codes for validation

[aybarsm]

It would be very useful to have a built-in configuration test feature that validates the configuration without starting the service.

Motivation:
Pre-deployment checks: Before rolling out changes to production, being able to test the configuration ensures that syntax and semantics are correct.

Automation workflows: Tools like Ansible, Puppet, or Chef often rely on a dry-run validation step to prevent pushing broken configurations. Without such a feature, automation is riskier and harder to guarantee correctness.

Proposed Behaviour:
A command or flag (e.g., --validate-config) that:

• Loads and validates the configuration.
• Exits with code 0 if the configuration is valid.
• Exits with a non-zero code if the configuration is invalid, ideally with a descriptive error message.

Benefits:

• Provides a safe mechanism to validate changes before deployment.
• Integrates seamlessly with CI/CD pipelines.
• Reduces downtime and misconfigurations by catching errors early.

Example usage:
sdns --config /etc/sdns.conf --validate-config

• Valid config → exits 0.
• Invalid config → exits 1 (or another non-zero value) and prints the reason.

[semihalev]

I will add this as soon as possible. Great option for sdns. I will inform again.

[semihalev]

I added this verification, release new version soon.