mxqd_control: Add new users at tail of user list

We are going to rely on the stability of the user list as a secondary
preference criteria to the number of jobs running in the cluster.

Put new user at the end of the user list so they start behind other
waiting users and will stay behing them if the main crtieria
(running_global_slot_count) is equal (e.g. zero).

mxqd_control.c

@@ -435,8 +435,11 @@ struct mxq_group_list *_server_add_group(struct mxq_server *server, struct mxq_g
 
     ulist->server = server;
 
-    ulist->next   = server->users;
-    server->users = ulist;
+    /* add new user at tail, so longer waiting users are preferred */
+    struct mxq_user_list **lastptr = &server->users;
+    while (*lastptr)
+        lastptr = &(*lastptr)->next;
+    *lastptr = ulist;
 
     server->user_cnt++;