Disallow user-created replication origins named "pg_xxx".
Since we generate such names internally, it seems like a good idea
to have a policy of disallowing them for user use, as we do for many
other object types. Otherwise attempts to use them will randomly
fail due to collisions with internally-generated names.
Discussion: https://postgr.es/m/3606.
1561747369@sss.pgh.pa.us