mplayer: keep pause state when frame stepping in last frame

When the last frame is displayed, and a frame step command is issued,
playback ands and advances to the next file. But before this commit,
the next file was played unpause. Fix this, and make sure pause is
kept.
mplayer: make --loop loop the playlist instead of each playlist entry

This is simpler and more useful. We could add a new switch for the old
functionality, but that would probably be more confusing than helpful.
When passing only a single file to the command line, this commit
shouldn't change behavior.

(Classic mplayer provided both features by duplicating the loop
functionality in the "playtree".)