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: https://postgr.es/m/
20180111064900.GA51030@paquier.xyz