Clean up group-finding in Gnus nnir search

Clean up group-finding in Gnus nnir search

This is part of removing code from nnir.el that isn't related to
searching backends and therefore belongs somewhere else.

* lisp/gnus/gnus-group.el (gnus-group-make-search-group)
(gnus-group-read-ephemeral-search-group): Put the logic for
determining the groups to search here, rather than in nnir. Improve
documentation.
* lisp/gnus/gnus-int.el (gnus-server-get-active): Renamed from
'nnir-get-active.
* lisp/gnus/nnir.el (nnir-run-imap, nnir-run-find-grep): Use it.
(nnir-get-active): Remove.
(nnir-make-specs): Make obsolete.
* lisp/gnus/nnselect.el (nnselect-group-server):  Make obsolete in
favor of 'gnus-group-server.