Make OFF keyword unreserved. It's not hard to imagine wanting to use 'off'

Make OFF keyword unreserved. It's not hard to imagine wanting to use 'off'
as a variable or column name, and it's not reserved in recent versions of
the SQL spec either. This became particularly annoying in 9.0, before that
PL/pgSQL replaced variable names in queries with parameter markers, so
it was possible to use OFF and many other backend parser keywords as
variable names. Because of that, backpatch to 9.0.