Add sanity check for pg_proc.provariadic

Check that the values from pg_proc.h match what ProcedureCreate would
have done.

Robert Haas and Amul Sul

Discussion: http://postgr.es/m/CA+TgmoZ_UGXfq5ygeDDMdUSJ4J_VX7nFnjC6mfY6BgOJ3qZCmw@mail.gmail.com