More intuitive chapter seek behavior

This implements the behavior I described [here](https://github.com/mpv-player/mpv/issues/173#issuecomment-22525929) yesterday; see that comment for a detailed-ish explanation.

I also fixed cycling the chapter property (it used to just close the file when cycling off the end in either direction). Probably not something that's going to be used very often, but there's no reason for it not to work.

This is my first time hacking on mpv, so I apologize if I missed any conventions you guys have. Let me know if anything needs to be changed (although I'm about to leave on vacation so I'll be slow to respond).