player: don't display zero duration for files with unknown duration

On OSD/terminal, just don't display the duration if unavailable.

Make the "length" property unavailable if duration is unavailable.
demux_mkv: export packet file position

This gives us approximate fallback playback percentage position if the
duration is unknown.
demux_mkv: implement percentage seeking with no index

It was implemented only for the case the index exists (pretty useless).
demux: move some seek flag sanitation to generic code

No reason why only demux_mkv.c should do this.
demux: fix demux_seek signature

Probably doesn't matter much.
input: cascade-load input.conf

If there are several input.confs in the set of valid config paths, load
them all.
vo_opengl: draw OSD twice in 3D mode case

Apparently this is needed for correct 3D mode subtitles. In general,
it seems you need to duplicate the whole "GUI", so it's done for all
OSD elements.

This doesn't handle the "duplication" of the mouse pointer. Instead,
the mouse can be used for the top/left field only. Also, it's possible
that we should "compress" the OSD in the direction it's duplicated, but
I don't know about that.

Fixes #1124, at least partially.