Introduce nnselect backend for gnus

Introduce nnselect backend for gnus

This new backend allows gnus to handle arbitrary sets of messages
spanning multiple groups, even when these groups are from different
backends and different servers. All gnus glue is removed from
nnir (leaving only the backend search functions) and gnus
search-related processing is done through nnselect. In appropriate
places 'nnir' has been replaced by 'nnselect' or 'search'.

* etc/NEWS: Document the change.
* doc/misc/gnus.texi: New  documentation for nnselect and update
searching and thread-referral  sections.
* lisp/gnus/nnselect.el: New file.
* lisp/gnus/nnir.el: Remove all gnus glue, leaving only searching
capability. Improve documentation strings.
* lisp/gnus/gnus-group.el (gnus-group-read-ephemeral-search-group,
gnus-group-make-search-group): New functions.
* lisp/gnus/gnus-msg.el (gnus-setup-message, gnus-group-news,
gnus-summary-news-other-window): Update to work for nnselect. Fix
gnus-newsgroup-name wrangling.
*lisp/gnus/gnus-registry.el
(gnus-registry-action,gnus-registry-ignore-group-p): Make work from nnselect.
* lisp/gnus/nnheader.el (nnheader-parse-head, nnheader-parse-nov):
Rework and consolidate header parsing.
* lisp/gnus/gnus-agent.el (gnus-agent-regenerate-group):
* lisp/gnus/gnus-cache.el (gnus-possibly-enter-article):
* lisp/gnus/gnus-cloud.el (gnus-cloud-available-chunks):
* lisp/gnus/gnus-msg.el (gnus-inews-yank-articles):
* lisp/gnus/gnus-sum. (gnus-get-newsgroup-headers):
* lisp/gnus/nndiary.el (nndiary-parse-head):
* lisp/gnus/nnfolder.el (nnfolder-parse-head):
* lisp/gnus/nnmaildir.el (nnmaildir--update-nov):
* lisp/gnus/nnml.el (nnml-parse-head):
* lisp/gnus/nnspool.el (nnspool-insert-nov-head):
Use new header parsing.
* lisp/gnus/gnus-start.el (gnus-read-active-for-groups): Rescan on
activation by default.
* lisp/gnus/gnus-sum.el (gnus-summary-line-format-alist): New specs
for virtual groups.
(gnus-article-sort-by-rsv, gnus-thread-sort-by-rsv):  New functions to
allow sorting by search RSV.