Add one-based OSD formating for playlist-pos

*I remember some conversation going on about something similar to this some time ago.*

Add an OSD formatted mode (?) to `playlist-pos` that indexes entries from one.
My main reason for wanting this is for use in `screenshot-template`.

Of course, other than what is displayed, nothing else is changed so command line arguments etc, are all still zero-based. This could potentially be unexpected to users.

I mostly just copied the code. I guess I could refactor it a bit...