pg_upgrade: Check for unsupported encodings.

pg_upgrade: Check for unsupported encodings.

Since we have dropped MULE_INTERNAL, add a check that all encodings used
in the source cluster are still supported according to
PG_ENCODING_BE_VALID().  This is done generically, in case we decide to
drop another encoding some day.

Suggested-by: Jeff Davis <pgsql@j-davis.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CA%2BhUKGKXDXh-FdU0orjfv%2BF08f%3DD91BhV3Ra-4zL-q%2BJmGYqTA%40mail.gmail.com