demux_playlist: detect headerless m3u files by extension

m3u files are normally just text files with a list of filenames. Nothing
actually mandates that there is a header. Until now, we've rejected such
files, because there's absolutely no way to detect them.

If nothing else claims the file, the extension is ".m3u", and if the
contents of the file look like text, then load it as m3u playlist. The
text heuristic is pretty cheap, but at least it should prevent trying
to load binary data as playlist. (Which would "work", but result in a
catastrophic user experience.)

Due to the text heuristic, UTF-16/32 files will be rejected (unless they
have a header), but I don't care.

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