ping: install a pong handler

Respond to server pings:

https://xmpp.org/extensions/xep-0199.html#s2c

```
xmpp DEBUG RECV: <iq id="rr-1694698253331-17375542133555098656-wf3J7kbG6j8Ft5Vx0cMY99txsW8=-55238004" type="get" to="msantos@example.com/110052433108464573191602" from="example.com"><ping xmlns="urn:xmpp:ping"/></iq>
conn DEBUG SENT: <iq id="rr-1694698253331-17375542133555098656-wf3J7kbG6j8Ft5Vx0cMY99txsW8=-55238004" to="example.com" type="result" from="msantos@example.com/110052433108464573191602"/>
```

Thanks @jessiehowell !

Fixes https://github.com/msantos/xmppipe/issues/9

src/ping.c

@@ -1,4 +1,4 @@
-/* Copyright (c) 2015-2019, Michael Santos <michael.santos@gmail.com>
+/* Copyright (c) 2015-2023, Michael Santos <michael.santos@gmail.com>
  *
  * Permission to use, copy, modify, and/or distribute this software for any
  * purpose with or without fee is hereby granted, provided that the above
@@ -44,3 +44,34 @@ void xmppipe_ping(xmppipe_state_t *state) {
 
   state->keepalive_fail++;
 }
+
+// <iq from='juliet@capulet.lit/balcony' to='capulet.lit' id='s2c1'
+// type='result'/>
+int handle_pong(xmpp_conn_t *const conn, xmpp_stanza_t *const stanza,
+                void *const userdata) {
+  xmppipe_state_t *state = userdata;
+  xmpp_stanza_t *iq;
+  const char *from;
+  const char *id;
+
+  from = xmpp_stanza_get_attribute(stanza, "from");
+  if (from == NULL)
+    return 1;
+
+  id = xmpp_stanza_get_attribute(stanza, "id");
+  if (id == NULL)
+    return 1;
+
+  iq = xmppipe_stanza_new(state->ctx);
+  xmppipe_stanza_set_name(iq, "iq");
+  xmppipe_stanza_set_type(iq, "result");
+  xmppipe_stanza_set_id(iq, id);
+  xmppipe_stanza_set_attribute(iq, "from",
+                               xmpp_conn_get_bound_jid(state->conn));
+  xmppipe_stanza_set_attribute(iq, "to", from);
+
+  xmppipe_send(state, iq);
+  (void)xmpp_stanza_release(iq);
+
+  return 1;
+}

src/xmppipe.c

@@ -346,6 +346,8 @@ int xmppipe_stream_init(xmppipe_state_t *state) {
   xmpp_handler_add(state->conn, handle_version, "jabber:iq:version", "iq", NULL,
                    state);
   xmpp_handler_add(state->conn, handle_iq, NULL, "iq", "result", state);
+  xmpp_handler_add(state->conn, handle_pong, "urn:xmpp:ping", "iq", "get",
+                   state);
   xmpp_id_handler_add(state->conn, handle_ping_reply, "c2s1", state);
 
   /* XXX multiple handlers can be called for each event

src/xmppipe.h

@@ -26,7 +26,7 @@
 #include "strtonum.h"
 #endif
 
-#define XMPPIPE_VERSION "0.14.9"
+#define XMPPIPE_VERSION "0.15.0"
 #define XMPPIPE_RESOURCE "xmppipe"
 
 #define XMPPIPE_STREQ(a, b) (strcmp((a), (b)) == 0)
@@ -111,6 +111,7 @@ void event_loop(xmppipe_state_t *state);
 int handle_message(xmpp_conn_t *const, xmpp_stanza_t *const, void *const);
 int handle_null(xmpp_conn_t *const, xmpp_stanza_t *const, void *const);
 int handle_ping_reply(xmpp_conn_t *const, xmpp_stanza_t *const, void *const);
+int handle_pong(xmpp_conn_t *const, xmpp_stanza_t *const, void *const);
 int handle_presence(xmpp_conn_t *const, xmpp_stanza_t *const, void *const);
 void xmppipe_ping(xmppipe_state_t *);
 int handle_presence_error(xmpp_conn_t *const, xmpp_stanza_t *const,