configure: fix some descriptions in the help output
mplayer: don't let playback resume force options that are file local

This happens by default with pausing: if a file was paused when doing
quit_watch_later, then resume and unpause, then the file played after
that would start in paused mode. This is because the pause option is
backed up at thr wrong place, so it backs up the state from resuming,
instead of whatever it was set to before that.
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.)