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

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.