[misc.lua] Restart mpv: Save position only of the current opened file

[KonoVitoDa]

If I use restart-mpv (my input.conf entry is Ctrl+e script-message-to misc restart-mpv) on a given file which is at 01:30, when I go to the next/previous file on the playlist, they will also start from 01:30, which is not the desired behavior.