This fixes "m:", "l:", "f:", "t:", "c:", "dfn:", and "n:" search
prefixes under indexlevel=medium when mixed with indexlevel=full
inboxish. We need positional data for Message-IDs, List-Id,
email addresses and filenames for exact matches, though we still
want to support wildcards.
Fortunately the storage cost is still small as these prefixes
tend to be small compared to message bodies. These are NOT
boolean terms since wildcard support and partial matching is
desired.