Various fixes and backward compatibility for gnus-search

Various fixes and backward compatibility for gnus-search

* lisp/gnus/gnus-group.el (gnus-group-make-search-group,
gnus-group-read-ephemeral-search-group): Check for and accept the old
nnir-* spec keys.
* lisp/gnus/gnus-search.el (shared-initialize): Use
generate-new-buffer instead of doing it ourselves.
(gnus-search-server-to-engine): Raise an informative error explicitly
if we can't find a search engine, rather than letting it fall through
to something less helpful.
(gnus-search-make-spec): Add `gnus-search--complete-key-data' to
`completion-at-point-functions' locally, not globally.