Refactor ExecGrant_*() functions

Instead of half a dozen of mostly-duplicate ExecGrant_Foo() functions,
write one common function ExecGrant_generic() that can handle most of
them.  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.

Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Antonin Houska <ah@cybertec.at>
Discussion: https://www.postgresql.org/message-id/flat/22c7e802-4e7d-8d87-8b71-cba95e6f4bcf%40enterprisedb.com

Author: petere