Re: allow granting CLUSTER, REFRESH MATERIALIZED VIEW, and REINDEX

Furthermore, MAINTAIN privileges on the partitioned table do not grant
the ability to create new partitions. There's a comment in tablecmds.c
alluding to a possible "UNDER" privilege:

  /*                                                                 
   * We should have an UNDER permission flag for this, but for now,   
   * demand that creator of a child table own the parent.             
   */

Perhaps there's something we want to do there, but it's a different use
case than the MAINTAIN privilege, so I don't see a reason it should be
grouped. Also, there's a bit of weirdness to think about in cases where
another user creates (and owns) a partition of your table (currently
this is only possible if the other user is a superuser).