The same parser was used for both preferred content expressions and
annex.largefiles. Reworked that today, splitting it into two distinct
parsers. It doesn't make any sense to use terms like "standard" or
"lackingcopies" in annex.largefiles, and such are now rejected.

That groundwork also let me add a feature that only makes sense for
annex.largefiles, and not for preferred content expressions: Matching by
mime type, such as `mimetype=text/*`
