libavformat/segment: change segment_list_size behavior to match hls_list_size behavior.

Make the segment muxer keep segment_list_size segments instead of
segment_list_size + 1 segments. This patch also changes the
documentation for segment_list_size to reduce possible confusion over
how many segments are kept.

this allows the segment list to
be limited to containing only one segment which used to be impossible
because a segment_list_size of 0 kept all the segments and a
segment_list_size of 1 kept 2 segments.

Signed-off-by: Simon Thelen <ffmpeg-dev@c-14.de>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>