mov: use tkhd enabled flag to set the default track

Signed-off-by: Anton Khirnov <anton@khirnov.net>
movenc: Make tkhd "enabled" flag QuickTime compatible

QuickTime will play multiple audio tracks concurrently if this flag is
set for multiple audio tracks.  And if no subtitle track has this flag
set, QuickTime will show no subtitles in the subtitle menu.

Signed-off-by: Anton Khirnov <anton@khirnov.net>