searchidx: store indexlevel=medium as metadata

And use it from Admin.

It's easy to tell what indexlevel=basic is from unconfigured
inboxes, but distinguishing between 'medium' and 'full' would
require stat()-ing position.* files which is fragile and
Xapian-implementation-dependent.

So use the metadata facility of Xapian and store it in the main
partition so Admin tools can deal better with unconfigured
inboxes copied using generic tools like cp(1) or rsync(1).

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