make --sub-auto and --audio-file-auto possible to autoload but do not prefer external tracks

The "do not select" here is NOT "strictly not selecting" (as what has been done to --external-files recently). This change simply makes auto-loaded external tracks seen as if they are internal tracks (but with higher tid than real internal ones) when selecting the track.~~

I think this behavior sounds more natural because when everything set to automatic, I don't really care which of the tracks are internal and which are external.

#5127 is probably saying the same thing. When auto-loading external files, all of the internal tracks with desired *slang will completely be ignored, because external has very high priority. IMO external should have such high priority when user specify external tracks with --sub-file or --audio-file, but when using auto-loading, it is most likely that users just want to make every track selectable.

There are external files not because users themselves made those files (and even want those files have higher priority), but because users simply got a bunch of files somewhere and simply want to play them.

EDIT: see my second comment below.

Start by examining mpv's track-selection handling for --sub-auto and --audio-file-auto, then compare the requested behavior with --external-files and issue #5127. Done means auto-loaded external tracks remain selectable without being preferred over matching internal tracks, while explicitly specified external tracks retain their priority.