Patch is addition to my previous patch
(https://lists.ffmpeg.org/pipermail/ffmpeg-cvslog/2012-June/051590.html)
and disables seeking on FIFOs/named pipes by setting
URLContext::is_streamed (same as pipe: protocol does for stdin/stdout pipes)

Fixes Ticket986
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>