Disable SSL 2.0 and session cache in OpenSSL driver

SSL 2.0 has security flaws and is deprecated for many years.
Session cache currently does not work anyway (as new SSL
context is made for every connection) so disable it.
Additionaly, instruct OpenSSL to free network buffers on idle
connections (this is available only in OpenSSL 1.0.0).

c_src/exmpp_tls_openssl.c

@@ -574,6 +574,24 @@ init_library(struct exmpp_tls_openssl_data *edd,
 		goto err;
 	}
 
+	// SSL 2.0 is deprecated for many years
+	SSL_CTX_set_options(edd->ctx, SSL_OP_NO_SSLv2);
+
+	/*
+	 * Since sessions are cached in SSL_CTX and currently new context
+	 * is used for every connection, then session caching makes little
+	 * sense, turn it off.
+	 */
+	SSL_CTX_set_session_cache_mode(edd->ctx, SSL_SESS_CACHE_OFF);
+	SSL_CTX_set_options(edd->ctx, SSL_OP_NO_TICKET);
+#ifdef SSL_MODE_RELEASE_BUFFERS
+	/*
+	 * This appeared in OpenSSL 1.0.0,
+	 * reduces memory usage on idle connections.
+	 */
+	SSL_CTX_set_mode(edd->ctx, SSL_MODE_RELEASE_BUFFERS);
+#endif
+
 	/* Set our certificate. */
 	if (edd->certificate != NULL) {
 		ret = SSL_CTX_use_certificate_chain_file(edd->ctx,