sound extract century/millennium date_part

Dear patchers,

Please find a small patch to fix the brain damage "century" and
"millennium" date part implementation in postgresql, both in the code and
the documentation, so that it conforms to the official definition. If you
do not agree with the official definition, please send your complaint to
"pope(at)vatican(dot)org". I'm not responsible for them;-)

With the previous version, the centuries and millenniums had a wrong
number and started the wrong year. Moreover century number 0, which does
not exist in reality, lasted 200 years. Also, millennium number 0 lasted
2000 years.

If you want postgresql to have it's own definition of "century" and
"millennium" that does not conform to the one of the society, just give
them another name. I would suggest "pgCENTURY" and "pgMILLENNIUM";-)

IMO, if someone may use the options, it means that postgresql is used for
historical data, so it make sense to have an historical definition. Also,
I just want to divide the year by 100 or 1000, I can do that quite easily.

Have a nice day,

--
Fabien Coelho - coelho(at)cri(dot)ensmp(dot)fr