Document "sctp" protocol

Reviewed-by: Stefano Sabatini <stefasab@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avformat/libmodplug: Reduce the probe score for small input

This ensures that theres enough data for mpeg_probe() to recognize mpeg-ps
Fixes Ticket2583

Based on code by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>