pgsql: Prevent extensions from creating custom GUCs that areGUC_LIST_Q

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/846b5a525746b83813771ec4720d664408c47c43

Modified Files
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src/backend/utils/misc/guc.c | 9 +++++++++
1 file changed, 9 insertions(+)