stream_dvb: Move PID-string parsing into a separate function, reduces code-duplication. Drop TPID-parsing again since teletext-stream seems not really correctly identified (ffmpeg guessed mp3 in my test...).
Parsing now done using strtok + sscanf instead of sscanfs with fixed numbers of expected fields - gives more correct checking of PID-overflow, will allow VDR-style PID-field parsing.