3.0 regression: UTF-16 subtitles no longer work

As reported on vlc-devel, UTF-16 subtitles no longer work. This is probably caused by:

commit c20bdd8d9238a8f3912a341df3a5f413324669e0
Author: Francois Cartegnie <fcvlcdev@free.fr>
Date:   Thu Sep 10 22:54:33 2015 +0200

    demux: subtitle: dont break non-seekable streams

which interferes with BOM detection in vlc_stream_ReadLine().

Reference: https://mailman.videolan.org/pipermail/vlc-devel/2016-November/110249.html

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