Better, faster, prettier groups_find_by_group_name

* there are really two operation modes that could even be put into separate
functions. So disentangle the logic and put the operation mode check
outside of the loop parsing the groups list.

src/group.c

···
 {
   rp_group *cur;
 
-  list_for_each_entry (cur, &rp_groups, node)
+  if (!exact_match)
+    {
+      list_for_each_entry (cur, &rp_groups, node)
+        {
+          if (cur->name && str_comp (s, cur->name, strlen (s)))
+            return cur;
+        }
+    }
+  else
     {
-      if (cur->name)
+      list_for_each_entry (cur, &rp_groups, node)
         {
-          if ((!exact_match && str_comp (s, cur->name, strlen (s))) ||
-              (exact_match && !strcmp (cur->name, s)))
+          if (cur->name && !strcmp (cur->name, s))
             return cur;
         }
     }