Fix parent position in RnNames.nubAvails

- `RnNames.nubAvails', which amalgamates AvailInfo items that belong to the 
  same parent, needs to be careful that the parent name occurs first if it is
  in the list of subnames at all.  (Otherwise, we can get funny export items
  in ifaces.)
- I discovered this while debugging family import/exports, but I am pretty 
  sure the bug would also have shown up without using families under the 
  right circumstances.