Initial landing of gnus nnselect backend

Initial landing of gnus nnselect backend

This is a new virtual backend for gnus, wherein any collection of
articles can be viewed as a gnus group (permanent or ephemeral).

* lisp/gnus/nnselect.el: New file.
* lisp/gnus/nnir.el: Remove the nnir backend but leave the search
functions.
* lisp/gnus/nnimap.el: Replace nnir backend related items with
nnselect.
(gnus-refer-thread-use-search): Renamed from gnus-refer-thread-use-nnir
(nnselect-search-thread): New function.
(nnimap-request-thread): Use it.
* lisp/gnus/gnus-group.el (gnus-group-make-search-group): New function
replacing gnus-group-make-nnir-group.
* lisp/gnus/gnus-msg.el: Replace nnir backend related items with
nnselect.
(gnus-setup-message): Pass virtual group article number to
gnus-inews-add-send-actions.
* lisp/gnus/gnus-registry.el (gnus-registry-action): Find the
originating article group when in an nnselect group.
(gnus-registry-ignore-group-p): Ignore virtual groups.
* lisp/gnus/gnus-srvr.el (gnus-group-make-search-group): Use new
function.
* lisp/gnus/gnus-sum.el (nnselect-article-): Use new nnselect backend
functions.
(gnus-summary-line-format-alist): Rework specs specific to nnselect
groups.
(nnselect-artlist):
(gnus-summary-local-variables): A new group-local variable.