avformat/hlsenc: add a use_localtime option to expand the segment filename with localtime

test examples:

./ffmpeg -re -i ~/Movies/objectC/facebook.mp4 -v verbose -c copy -f hls -hls_segment_filename test-%s.ts -use_localtime 1 -bsf:v h264_mp4toannexb aaa.m3u8

[StevenLiu@localhost ffmpeg]$ cat aaa.m3u8;ll test-*.ts
test-1441052221.ts
test-1441052231.ts
test-1441052235.ts
test-1441052243.ts
test-1441052249.ts
-rw-r--r--  1 StevenLiu  staff  1310736  9  1 04:15 test-1441052131.ts
-rw-r--r--  1 StevenLiu  staff   495192  9  1 04:15 test-1441052141.ts
-rw-r--r--  1 StevenLiu  staff  1310736  9  1 04:17 test-1441052212.ts
-rw-r--r--  1 StevenLiu  staff  1067840  9  1 04:17 test-1441052221.ts
-rw-r--r--  1 StevenLiu  staff   235564  9  1 04:17 test-1441052231.ts
-rw-r--r--  1 StevenLiu  staff  1187220  9  1 04:17 test-1441052235.ts
-rw-r--r--  1 StevenLiu  staff   694848  9  1 04:17 test-1441052243.ts
-rw-r--r--  1 StevenLiu  staff   526588  9  1 04:17 test-1441052249.ts
[StevenLiu@localhost ffmpeg]$

./ffmpeg -re -i ~/Movies/objectC/facebook.mp4 -v verbose -c copy -f hls -hls_segment_filename test-%s.ts -use_localtime 1 -bsf:v h264_mp4toannexb aaa.m3u8

[StevenLiu@localhost ffmpeg]$ cat aaa.m3u8;ll aaa-*.ts
aaa-1441052417.ts
aaa-1441052427.ts
aaa-1441052437.ts
aaa-1441052440.ts
aaa-1441052449.ts
-rw-r--r--  1 StevenLiu  staff  1310736  9  1 04:19 aaa-1441052382.ts
-rw-r--r--  1 StevenLiu  staff   277300  9  1 04:19 aaa-1441052392.ts
-rw-r--r--  1 StevenLiu  staff  1310736  9  1 04:20 aaa-1441052417.ts
-rw-r--r--  1 StevenLiu  staff  1067840  9  1 04:20 aaa-1441052427.ts
-rw-r--r--  1 StevenLiu  staff   235564  9  1 04:20 aaa-1441052437.ts
-rw-r--r--  1 StevenLiu  staff  1187220  9  1 04:20 aaa-1441052440.ts
-rw-r--r--  1 StevenLiu  staff   338776  9  1 04:20 aaa-1441052449.ts
[StevenLiu@localhost ffmpeg]$

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