mplayer: make --save-position-on-quit save only on quit

When enabling --save-position-on-quit, playback position stored not only
on quit, but in any case playback of a file was stopped. This includes
going to the next file with playlist navigation commands.

After some discussion on IRC, it turned out that nobody thought this was
good behavior. Disable it, and really make it save only on quit.

Maybe the option is useless now, as the user could remap the CLOSE_WIN
key binding. On the other hand, CLOSE_WIN sounds and _is_ a bit obscure.
mplayer: print informative message when resuming playback

In particular, this informs the user how to disable this.