Passing SSL_VERIFY_PEER makes OpenSSL shut down the connection if the
certificate is invalid, without giving us a chance to ignore that
error. Pass SSL_VERIFY_NONE and call SSL_get_verify_result if the user
wanted us to check.

When no CNs match, we used to jump to on_error which gave a bogus
error as that's for OpenSSL errors. Jump to cert_fail so we tell the
user that the error came from checking the certificate.