Make DTS probe more robust against false positives (as e.g. probetest shows).

Make DTS probe more robust against false positives (as e.g. probetest shows).
In particular check that the detected markers clearly indicate a specific DTS
format (a wild mixture of e.g. little- and big-endian markers is unlikely to be
a valid DTS file) and ensure the markers appear with sufficient frequency.

Originally committed as revision 19844 to svn://svn.ffmpeg.org/ffmpeg/trunk