This patch simply adds demuxing support for AC-3 streams in DVB TS

This patch simply adds demuxing support for AC-3 streams in DVB TS
files.  In these streams AC-3 is given type 0x06 (private stream) and
includes a descriptor with either tag 0x6A or 0x7A.  The code already
handles ATSC AC-3 which uses stream type 0x81.  All the new patch does
is look for any stream type 0x06 that has a descriptor with the proper
tag (i.e. 0x6A or 0x7A) and if found forces it to be recognized as an
AC-3 stream.
From Andy Brown <abrown at daqtron com>

Originally committed as revision 5260 to svn://svn.ffmpeg.org/ffmpeg/trunk