Fix Gnus group sorting to use gnus-group-list

Fix Gnus group sorting to use gnus-group-list

* lisp/gnus/gnus-group.el (gnus-group-sort-flat): As
  gnus-newsrc-hashtb is now a real (unsorted) hash table, use
  gnus-group-list to maintain group sort order.
  (gnus-group-sort-selected-flat): Ditto.
* lisp/gnus/gnus-start.el (gnus-subscribe-alphabetically): Simplify
  function using seq-find.
  (gnus-subscribe-killed, gnus-subscribe-zombies): Use cl-pushnew to
  avoid adding duplicates (can happen when un/subscribing multiple
  times to one group).