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