Rename osd_chan_name -> osd_display

Rename this variable to be more generic as we will be displaying more
things than just the channel name with xosd. The first thing will be to
notify the status of subtitles.

Signed-off-by: Andrew Clayton <andrew@digital-domain.net>
Display status of subtitles

When enabling/disabling subtitles we now display; Subtitles: On,
Subtitles: Off or Subtitles: None. The last one is displayed when we
tried to enable subtitles but none were found.

Signed-off-by: Andrew Clayton <andrew@digital-domain.net>