conn: clear system handlers on reconnect

xmpp_conn_t object can be reused with saving user's handlers.
However, saving system handlers can lead to a fail during connection
process. This is because old object may contain timed handlers for
missed features or other handlers that would handle incoming stanzas
incorrectly.

src/common.h

@@ -266,6 +266,7 @@ void handler_add(xmpp_conn_t * const conn,
                  const char * const name,
                  const char * const type,
                  void * const userdata);
+void handler_system_delete_all(xmpp_conn_t *conn);
 
 /* utility functions */
 void disconnect_mem_error(xmpp_conn_t * const conn);