New gnus-search library

New gnus-search library

This library provides a fundamental reworking of the search
functionality previously found in nnir.el.  It uses class-based search
engines to interface with external searching facilities, and a parsed
search query syntax that can search multiple engines.

* lisp/gnus/gnus-search.el: New library containing search
functionality for Gnus.
* doc/misc/gnus.texi: Document.
* lisp/gnus/gnus-group.el (gnus-group-make-search-group,
gnus-group-read-ephemeral-search-group): Remove references to nnir,
change meaning of prefix argument, change values of nnselect-function
and nnselect-args.
* lisp/gnus/nnselect.el: Replace references to nnir
(nnselect-request-article): Use gnus-search functions, and search
criteria.
(nnselect-request-thread, nnselect-search-thread): Use gnus-search
thread search.
(gnus-summary-make-search-group): Switch to use gnus-search function
and arguments.
* test/lisp/gnus/search-tests.el: Tests for new functionality.