util.xmppstream: Send <?xml?> declaration in same call as stream open If network_settings.opportunistic_writes is enabled then this would previously have resulted in two socket writes, and possibly two packets being sent. This caused some issues in older versions of Gajim, which apparently expected the stream opening in the first packet, and thus it could not connect. With this change and opportunistic_writes enabled, the first packet should contain both the xml declaration and the stream open tag. Without opportunistic_writes, there should be no observable change. Tested with Gajim 1.1.2 (on same machine). Unsure if loopback behaves differently than the network here.