Fix errors when nnselect-always-regenerate is t (bug#61539)

Fix errors when nnselect-always-regenerate is t (bug#61539)

The group parameter nnselect-always-regenerate causes the list of
articles in the group to be generated each time it is needed. For this
to work reliably the list of articles has to be generated at the
appropriate time and to have a reproducible ordering.

* lisp/gnus/gnus-search.el (gnus-search-run-search): For nnselect
groups if the article list has not been stored, regenerate it.
* lisp/gnus/nnselect.el (nnselect-generate-artlist): Sort the
generated list of articles by RSV, group, and number.  Store the
artlist after generation. When the new optional argument INFO is
non-nil, update the group info.
(nnselect-compress-artlist, nnselect-uncompress-artlist): Preserve the
article list ordering.
(nnselect-get-artlist): Return nil when nnselect-always-regenerate is t.
(nnselect-store-artlist): Store the group active range along with the
artlist. Don't keep the artlist if nnselect-always-regenerate is t.
(nnselect-request-group): The full article list isn't needed at this
stage, only the active range.
(nnselect-retrieve-headers): Regenerate the article list if there
is no stored value. Inhibit gnus-demon while retrieving headers.
(nnselect-request-group-scan): Don't generate the article list when
nnselect-always-regenerate is t since it will be generated again later
on.
(nnselect-request-create-group): Allow the artlist to be passed as an
argument to the function. Update the group info and store the artlist.