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+  If your life is a hard drive,     |  830 Blythe Avenue +  Christ can be your backup.        |  Drexel Hill,
Pennsylvania19026