Prevent extensions from creating custom GUCs that are GUC_LIST_QUOTE.

Pending some solution for the problems noted in commit 74286994,
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

src/backend/utils/misc/guc.c

@@ -7606,6 +7606,15 @@ init_custom_variable(const char *name,
 		!process_shared_preload_libraries_in_progress)
 		elog(FATAL, "cannot create PGC_POSTMASTER variables after startup");
 
+	/*
+	 * We can't support custom GUC_LIST_QUOTE variables, because the wrong
+	 * things would happen if such a variable were set or pg_dump'd when the
+	 * defining extension isn't loaded.  Again, treat this as fatal because
+	 * the loadable module may be partly initialized already.
+	 */
+	if (flags & GUC_LIST_QUOTE)
+		elog(FATAL, "extensions cannot define GUC_LIST_QUOTE variables");
+
 	/*
 	 * Before pljava commit 398f3b876ed402bdaec8bc804f29e2be95c75139
 	 * (2015-12-15), two of that module's PGC_USERSET variables facilitated