Make pg_get_ruledef() parenthesize negative constants (Tom)

Before this fix, a negative constant in a view or rule might be dumped as, say, -42::integer, which is subtly incorrect: it should be (-42)::integer due to operator precedence rules. Usually this would make little difference, but it could interact with another recent patch to cause PostgreSQL to reject what had been a valid SELECT DISTINCT view query. Since this could result in pg_dump output failing to reload, it is being treated as a high-priority fix. The only released versions in which dump output is actually incorrect are 8.3.1 and 8.2.7.