lib: make folder: prefix literal

In xapian terms, convert folder: prefix from probabilistic to boolean
prefix, matching the paths, relative from the maildir root, of the
message files, ignoring the maildir new and cur leaf directories.

folder:foo matches all message files in foo, foo/new, and foo/cur.

folder:foo/new does *not* match message files in foo/new.

folder:"" matches all message files in the top level maildir and its
new and cur subdirectories.

This change constitutes a database change: bump the database version
and add database upgrade support for folder: terms. The upgrade also
adds path: terms.

Finally, fix the folder search test for literal folder: search, as
some of the folder: matching capabilities are lost in the
probabilistic to boolean prefix change.