paplayer: fix mp4 playback

with the AirPlay fix I reverted the eof change in input stream which tuned out NOT to be responsible for broken YouTube app vie AirPLay https://github.com/quasar1/boxeebox-xbmc/commit/246d78614ead21795ae1abe3cc1151f004e5105f#commitcomment-6191602

paplayer needs to detect if a seek is possible. the alternative method of resetting the flag by seekin the input stream directly made mp4 files fail.

- tested Airplay with iTunes
- tested flac
- wma losless (not seekable)
- mp4
- mp3
- ac3
- dts
- ogg

@da-anda could you verify please