Better configuration validation

[mcpherrinm]

Right now, we allow some conflicting configuration values. Like you can configure a unix socket and a TLS certificate and key, which will be ignored. Or if you configure just a certificate, you'll get an error loading the key.

We should be more explicit about what options are required, and not allow "useless" flags.

If a unix:// url is provided, we could require an empty host and nonempty path

[pgporada]

Example of "If a unix:// url is provided, we could require an empty host and nonempty path"
https://go.dev/play/p/_xjiXUWreOF