If the user has LEFT/RIGHT/etc. bound in his input.conf, then these were
overriding the menu keys in dvdnav mode.
This hack works because the dvdnav crap happens to be the only user of
MP_INPUT_ON_TOP. If it finds a default key binding in the dvdnav menu
section, it will use that, instead of continuing search and possibly
finding the user key bindings meant for normal playback.