Extend DV autodetection to also reliably detect single-frame DVs with
a higher score that MAX/4.
It checks that there are at least 10 DIF headers and at least one per
24000 bytes, and if so considers the file reliably detected as DV.
Passes probetest, too.

Originally committed as revision 20074 to svn://svn.ffmpeg.org/ffmpeg/trunk
