dvb_support: add dvb_mux_conf_str()
dvb autodiscovery: more logs and improvements

- added logs for the scanfile mux creation
- added logs for the autodiscovery mux creation
- added logs for the autodiscovery mux changes
- use also symbol rate for the equal mux comparison
- handle the delivery system change (dvb-s / dvb-s2 switch for example)