player: fix initial selection with --secondary-sid

Also, make sure that a track can't be selected twice. While this might
work in some situations, it certainly won't work with subtitles demuxed
from a stream.

Fixes #452.
player: deselect secondary sub when switching to file with different tracks

This applies the usual logic of resetting stream selections to default
when switching to a file with a different track layout. (This is to
prevent selecting random streams.)