Fix handling of CC (century) format spec in to_date/to_char. According to

standard convention the 21st century runs from 2001-2100, not 2000-2099,
so make it work like that.  Per bug #2885 from Akio Iwaasa.

Backpatch to 8.2, but no further, since this is really a definitional
change; users of older branches are probably more interested in stability.