Remove nnselect-artlist variable in nnselect

Remove nnselect-artlist variable in nnselect

This fixes a bug which causes the group info to get out of sync.

* lisp/gnus/nnselect.el (nnselect-add-prefix): New macro.
  (nnselect-get-artlist): New macro.
  (nnselect-possibly-change-group): Delete.
  (nnselect-request-group, nnselect-retrieve-headers,
  nnselect-request-update-info, nnselect-close-group): Remove
  nnselect-artlist variable and update with new macros.
  (nnselect-request-update-info): Use memq instead of member for speed.