I rely on nnir so I thought I would just try to get it working with
minimal rewriting.  I haven't tested this much, but it worked on a few
simple searches. The main change was to nnir-run-imap, but I also had to
make one change to nnimap-retrieve-headers that worries me. Currently
this function returns 't, and nnir wants it to return 'headers. I made
the change and nothing exploded. Is this OK?

Of course all this needs to be rewritten, but in the meantime...