core/command: allow seek to "chapter -1"

This will seek to the start of the file regardless of whether the first real
chapter starts there or not.
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 error out on attempts to cycle the chapter property. It didn't work right
and fixing it properly would have added too much complexity, seeing as nobody
seems to use it.
manpage: --chapter-seek-threshold