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
d363d42bb9a4399a0207bd3b371c966e22e06bd3, 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