[omxplayer] Make use of TrueHD fastpath when downmixing

The TrueHD codec actually works in 3 stages. It decodes the downmixed stereo. It then decodes the differences required to produce 5.1.
It then decodes the differences required to produce 7.1.

Many users end up downmixing this 7.1 stream back to 2.0.
Much better to tell the codec we only need the 2.0 stream. It saves about 50% of the CPU required
- ensure to announce airtunes before airplay via zeroconf (makes it more likely that ios7 clients detect us correctly at the first shot)
- add method to reannounce already published services
- reannounce airplay service every 10 secs (fixes ios7 clients not detecting us when their wlan was asleep or they saw the airplay announcement before the airtunes annoncement)