Fix for plpgsql and timestamp/interval

I found that plpgsql did not have the proper mapping of
datetime->timestamp and timespan->interval. It had the old 6.5.*
reverse mappings. Fix in CVS and will appear in 7.0.1.

-- 
  Bruce Momjian                        |  http://www.op.net/~candle
  pgman@candle.pha.pa.us               |  (610) 853-3000
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