After a MINVALUE/MAXVALUE bound, allow only more of the same.

In the old syntax, which used UNBOUNDED, we had a similar restriction,
but commit d363d42b, which changed the
syntax, eliminated it.  Put it back.

Patch by me, reviewed by Dean Rasheed.

Discussion: http://postgr.es/m/CA+Tgmobs+pLPC27tS3gOpEAxAffHrq5w509cvkwTf9pF6cWYbg@mail.gmail.com