Add ISOM_EDIT_DURATION_IMPLICIT for case where user has no interest in the
duration of an edit.

The duration is expected to be constructed by demuxer.
remuxer: Add --fragment to enable movie fragments.
remuxer: Output the empty initial movie when enabling movie framents.
remuxer: Fix fragment duration when the first input sample has non-zero DTS.
fragment: Fix a crash when freeing unneeded tfra entries.
remuxer: Fix possibility of an empty movie fragment at the beginning of output.
remuxer: Fix timing of fragment file mode setting.
Support writing of simple one-level Segment Index Boxes.

Hierarchical and daisy-chain indexing are not supported yet.