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.

ChangeLog

@@ -4,6 +4,8 @@
 	- OpenSSL tls module disables insecure SSLv2 SSLv3 and TLSv1
 	- Support of handlers with the same callback function, but different
 	  userdata
+	- System handlers are deleted on xmpp_conn_t reconnection. Old system
+	  handlers could cause problems
 	- New public function xmpp_sha1_digest()
 
 0.9.1