demux_mf: use stream API to open list files

mf:// has an obscure feature that lets you pass a list of filenames
separated by newlines. Who knows whether anyone is using that. It opened
these listfiles with fopen(), so the recent stream origin bullshit
doesn't operate on it. Fix this by using the mpv internal stream API
instead. Unfortunately there is no fgets(), so write an ad-hoc one. (An
implementation of line reading via "stream" is still in demux_playlist,
but it's better to keep it quarantined there.)

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