Support "allow" SSL-mode in server configuration

Would be nice for consistency with listener configuration. For servers, "ssl=allow" means try with SSL but fall back to unencrypted connections if server does not support SSL.

In the mariadbbackend protocol code, look at the server capabilities to see if SSL is possible. With ConnectorC-based connections, configure SSL but do not check that the final connection is encrypted.

Server certificates should not be verified unless the relevant settings are enabled. This is different from how the command line client operates (it verifies by default), but verifying certificates by default could cause issues for existing users with older server versions.