command: more intuitive chapter seek behavior

If close to chapter start, skipping back goes to previous chapter (no change).
If more than <threshold> seconds in, skipping back will now go to the beginning
of the current chapter instead.

The threshold is set by the new option --chapter-seek-threshold and defaults to
5 seconds.  A negative value disables the new functionality.

Also made cycle chapter work.
manpage: --chapter-seek-threshold