Set GUC "is_superuser" in all processes that set AuthenticatedUserId.

It was always false in single-user mode, in autovacuum workers, and in
background workers.  This had no specifically-identified security
consequences, but non-core code or future work might make it
security-relevant.  Back-patch to v11 (all supported versions).

Jelte Fennema-Nio.  Reported by Jelte Fennema-Nio.

src/backend/utils/init/miscinit.c

@@ -842,6 +842,14 @@ InitializeSessionUserIdStandalone(void)
 
 	AuthenticatedUserId = BOOTSTRAP_SUPERUSERID;
 	SetSessionUserId(BOOTSTRAP_SUPERUSERID, true);
+
+	/*
+	 * XXX This should set SetConfigOption("session_authorization"), too.
+	 * Since we don't, C code will get NULL, and current_setting() will get an
+	 * empty string.
+	 */
+	SetConfigOption("is_superuser", "on",
+					PGC_INTERNAL, PGC_S_DYNAMIC_DEFAULT);
 }
 
 /*