XEP-0410: implement server-side optimisation for self-pings

This prevents the ping from round-tripping through IRC and
possibly a random other client of the user. Please see XEP-0410
for the rationale.

Fixes #3385.

src/xmpp/biboumi_component.cpp

@@ -997,7 +997,7 @@ void BiboumiComponent::send_irc_channel_disco_info(const std::string& id, const
     identity["category"] = "conference";
     identity["type"] = "irc";
     identity["name"] = ""s + iid.get_local() + " on " + iid.get_server();
-    for (const char *ns: {DISCO_INFO_NS, MUC_NS, ADHOC_NS, PING_NS, MAM_NS, VERSION_NS, STABLE_MUC_ID_NS})
+    for (const char *ns: {DISCO_INFO_NS, MUC_NS, ADHOC_NS, PING_NS, MAM_NS, VERSION_NS, STABLE_MUC_ID_NS, SELF_PING_FLAG})
       {
         XmlSubNode feature(query, "feature");
         feature["var"] = ns;