Refactor recordExtObjInitPriv()

Another aclchk.c refactoring patch, similar to [0] and [1].

Refactor recordExtObjInitPriv():  Instead of half a dozen of 
mostly-duplicate conditional branches, write one common one that can 
handle most catalogs.  We already have all the information we need, such 
as which system catalog corresponds to which catalog table and which 
column is the ACL column.


[0]: 
https://www.postgresql.org/message-id/95c30f96-4060-2f48-98b5-a4392d3b6066@enterprisedb.com
[1]: 
https://www.postgresql.org/message-id/flat/22c7e802-4e7d-8d87-8b71-cba95e6f4bcf%40enterprisedb.com