avformat/hlsenc: add warning for append_list and hls_init_time option

When use append_list mode, the hls_init_time set nouse,
Because the append_list only support append at the old m3u8 end
cannot set init segments durations at the middle of the list.
That's invalid. and show a warning message for user.

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
