* Enable SSL on options ssl_cert_file or ssl_ca_file being set, as these make
  sense for client side connections, whilst a key does not.

* Remove non-client SSL_VERIFY options, as they do nothing as per:
  https://www.openssl.org/docs/ssl/SSL_CTX_set_verify.html,
SSL_VERIFY_FAIL_IF_NO_PEER_CERT and SSL_VERIFY_CLIENT_ONCE are options to
configure the behaviour of SSL_VERIFY_PEER. Thus they *must* be used with
SSL_VERIFY_PEER.

So we can automatically set SSL_VERIFY_PEER if either is requested. As usage of
these alone is invalid.

https://www.openssl.org/docs/ssl/SSL_CTX_set_verify.html