avformat/hlsenc: strftime identifiers and segment index

avformat/hlsenc: strftime identifiers and segment index
 in filenames

Putting date/time values into segment filenames is very usefull.
But to produce non-conflicting segment filenames with -use_localtime
option with date/time
values in hls_segment_filename option, sometimes is not enough.
Like in cases when multiple segments produced in the same second.
But hlsenc currently does not make possible to use segment index (%d) at
the

same time whe use_localtime is in effect, due to identifier conflict.
This patch makes possible to use strftime identifiers and still put
segment index (%d) at same time in segment filenames by introducing
second_level_segment_index flag. When -use_localtime is active,
identifier %d is for month day index, so %%d is the segment index
placeholder. This enhanced behaviour only exists when new
second_level_segment_index flag is specified.
For instance putting 'segment_%Y%m%d%H%M%S_%%05d.ts' value into
-hls_segment_filename option and specifing -hls_flags
second_level_segment_index and -use_localtime 1, may produce segment
filename as 'segment_20161230235758_00002.ts'

An example:

ffmpeg -loglevel info -y -f lavfi -i color=c=red:size=640x480:r=25 -f
lavfi -i anullsrc=r=44100:cl=stereo -c:v mpeg2video -g 25 -acodec aac
-cutoff 20000 -ac 2 -ar 44100 -ab 192k -f hls -hls_time 3 -hls_list_size
5 -hls_flags delete_segments+second_level_segment_index -use_localtime 1
-hls_segment_filename "segment_%Y%m%d%H%M%S_%%05d.ts" stream.m3u8

will produce segments filenames:

....
segment_20161227005902_00013.ts
segment_20161227005902_00014.ts
segment_20161227005902_00015.ts
segment_20161227005903_00016.ts
segment_20161227005903_00017.ts
segment_20161227005903_00018.ts
segment_20161227005903_00019.ts
segment_20161227005903_00020.ts
....

Signed-off-by: Bela Bodecs <bodecsb@vivanet.hu>