This allows the underlying files to change their duration on subsequent
avformat context opens.

An example use case where this matters:

ffconcat version 1.0
file dummy.mxf
file dummy.mxf

ffmpeg -re -stream_loop -1 -i dummy.ffconcat -f sdl2 none

The user can seamlessly change the input by atomically replacing dummy.mxf.

v2: Set ConcatFile duration in read_header for all segments with known
durations because from now on we always recalculate the start time in
open_file, and an instant seek could have caused unset ConcatFile durations.
Signed-off-by: Marton Balint <cus@passwd.hu>