avformat/hlsenc: add hls_flag option to write segments to temporary file until complete

avformat/hlsenc: add hls_flag option to write segments to temporary file until complete

Adds a `-hls_flags +temp_file` which will write segment data to
filename.tmp, and then rename to filename when the segment is complete.

This patch is similar in spirit to one used in Plex's ffmpeg fork, and
allows a transcoding webserver to ensure incomplete segment files are
never served up accidentally.

Reviewed-by: Hendrik Leppkes <h.leppkes@gmail.com>
Reviewed-by: Bodecs Bela <bodecsb@vivanet.hu>
Signed-off-by: Aman Gupta <aman@tmm1.net>
Signed-off-by: Steven Liu <lq@chinaffmpeg.org>