Refuse files with lines unexpectedly starting with a dash

We support dash-encoding even if we don't really work with files who
would need it as implementations are free to encode every line, but
otherwise a line starting with a dash must either be a header we parse
explicitly or the file is refused. This is against the RFC which says
clients should warn on such files, but given that we aren't expecting
any files with dash-started lines to begin with this looks a lot like a
we should not continue to touch the file as it smells like an attempt to
confuse different parsers by "hiding" headers in-between others.

The other slightly more reasonable explanation would be an armor header
key starting with a dash, but no existing key does that and it seems
unlikely that this could ever happen. Also, it is recommended that
clients warn about unknown keys, so new appearance is limited.

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