mplayer: handle --reset-on-next-file=""

The option list contains an empty string member with this option value,
so ignore that. I'm not sure whether the option list should maybe be
empty in this case, but it could be the wrong thing in case of other
options.
tv: attempt to support mjpeg streams

MPlayer handles this correctly, because MPlayer still has the FourCC
codec dispatch (codecs.conf). We need to handle this case specially,
because the libavformat rawvideo decoder will of course not eat mjpeg.

mjpeg is the only supported format, though. (Even MPlayer needs to
convert between V4L2 formats and MPlayer FourCCs, and mjpeg is the only
non-raw format.)