Apparently, libav doesn't have the change for the new way to create a
libavformat context merged yet. So, we can't use that...
Rather, this commit fixes format specific avoptions another way.
On the downside, invalid format options are now detected very late, and
any attempt to set an option value to +something or -something will
append to the previously set option value (this logic can no longer be
specific to options of bitflag type, as finding out the option type is
what we simply cannot do with this interface).
This allows to define which stream is to be used as first output stream.
This is useful because dvdauthor refuses VOB files where the audio
stream is the first stream.
