Fix version numbering foulups exposed by 10.1.

Fix version numbering foulups exposed by 10.1.

configure computed PG_VERSION_NUM incorrectly.  (Coulda sworn I tested
that logic back when, but it had an obvious thinko.)

pg_upgrade had not been taught about the new dispensation with just
one part in the major version number.

Both things accidentally failed to fail with 10.0, but with 10.1 we
got the wrong results.

Per buildfarm.