XML: Add "sslmethod" attribute to heartbleed element.

There may or may be not a custom OpenSSL build out there,
which is only vulnerable to Heartbleed via TLSv1.0, but not
via 1.1 or 1.2. Since the XML did not include this information,
there was no chance to automatically discover this
strangeness.

Also: simplified `SSL_METHOD`→string conversion.
add a few more `ifndef OPENSSL_NO_SSL3` and enable more warnings