Add aliases for recent Gnus nnselect changes

Add aliases for recent Gnus nnselect changes

* lisp/org/ol-gnus.el (org-gnus-store-link): Change 'nnir to
'nnselect.
* lisp/gnus/gnus-group.el: Define obsolete function alias for
'gnus-group-make-nnir-group to
'gnus-group-read-ephemeral-search-group.
* lisp/gnus/gnus-sum.el: Define obsolete variable alias for
'gnus-refer-thread-use-nnir to 'gnus-refer-thread-use-search.
* lisp/gnus/nnselect.el: Define obsolete variable alias for
'nnir-retrieve-headers-override-function to
'nnselect-retrieve-headers-override-function.
* lisp/gnus/nnir.el: Restore definition of 'nnir-summary-line-format
and mark obsolete.