Update log levels for Mutual TLS

[k0ekk0ek]

Currently errors related to certificates for Mutual TLS are logged via DEBUG. Which means they are not available in release builds. Use a method, like normal log at level 5 or so to allow for more convenient debugging. See #362 for more details.

[bilias]

Notes:

Some common certificate errors I believe we should log in normal logging (5 or something else) with an clear message explaining the exact problem to the operator:

• certificate CN/SAN mismatch with tls-auth auth-domain-name
• certificate expired
• tls-cert-bundle does not verify certificate given by client (server side)
• tls-cert-bundle does not verify certificate given by server (client side)
• possible connection errors on tls-auth-port
• maybe better output when client requests tls XFR on non tls-auth-port
• certificate/key open file errors or permissions problem/warning checks
• other certificate errors?

Maybe some of them are already covered, I just wanted to document all possible (common) scenarios.