os-bind: Validate configuration before saving and display errors

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Is your feature request related to a problem? Please describe.
When configuring bind through the web UI, even small misconfiguration can result in Bind rejecting the config. This information is not forwarded to the user and as long as Bind is not fully stopped and started, it will continue to run with the old config, further delaying identifying this problem. Depending on the problem, it can prevent Bind from starting after the router restarts, e.g. after a power outage or system update.

Describe the solution you'd like
The bind plugin should validate that the configuration is valid. Bind has CLI tools to do that, named-checkconf and named-checkzone. These should be invoked and the output forwarded to the web UI to have a proper feedback loop.

Describe alternatives you've considered
An alternative could be a check-config / check-zones button, but I believe this type of feedback should be part of the standard save action.

Start at the os-bind web UI save action and trace the configuration validation flow. Review the proposed named-checkconf and named-checkzone entry points, then confirm that their output reaches the UI and that invalid configurations cannot be saved without actionable feedback.