Prevent extensions from creating custom GUCs that are GUC_LIST_QUOTE.
Prevent extensions from creating custom GUCs that are GUC_LIST_QUOTE.

Pending some solution for the problems noted in commit 742869946,
disallow dynamic creation of GUC_LIST_QUOTE variables.

If there are any extensions out there using this feature, they'd not
be happy for us to start enforcing this rule in minor releases, so
this is a HEAD-only change.  The previous commit didn't make things
any worse than they already were for such cases.

Discussion: f8438015c7c7cf40f93023fd39ffe43a4b8d1810@paquier.xyz

adam8157/gpdb,50wu/gpdb,xinzweb/gpdb,adam8157/gpdb,greenplum-db/gpdb,50wu/gpdb,adam8157/gpdb,greenplum-db/gpdb,xinzweb/gpdb,adam8157/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,lisakowen/gpdb,xinzweb/gpdb,50wu/gpdb,adam8157/gpdb,adam8157/gpdb,50wu/gpdb,50wu/gpdb,lisakowen/gpdb,50wu/gpdb,xinzweb/gpdb,xinzweb/gpdb,lisakowen/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,xinzweb/gpdb,lisakowen/gpdb,50wu/gpdb,adam8157/gpdb,greenplum-db/gpdb,adam8157/gpdb,lisakowen/gpdb,xinzweb/gpdb,lisakowen/gpdb,50wu/gpdb,lisakowen/gpdb,lisakowen/gpdb,xinzweb/gpdb