Fixes the following warning:
libavutil/timecode.c:103:60: warning: '%02d' directive output may be truncated writing between 2 and 10 bytes into a region of size between 0 and 7
Some demuxers set a timecode in the format or streams metadata. The
muxers now make use of this metadata instead of a duplicated private
option.
This makes possible transparent copy of the timecode when transmuxing
and transcoding.
-timecode option for MPEG1/2 codec is also renamed to -gop_timecode. The
global ffmpeg -timecode option will set it anyway so no option change
visible for the user.
