Paths like foo.mkv, ./foo.mkv .//foo.mkv, ../"$(basename
"$PWD")"/foo.mkv, and C:\foo.mkv and C:/foo.mkv on Windows, use
different config files for resuming playback, so if you quit-watch-later
and later play the same file with a different path, mpv does not resume
playback. This commit normalizes the paths on Unix to fix this.