information_schema.columns changes needed for OLEDB

As we discussed at pgcon2009 there are some changes/fixes necessary in
information_schema.columns to allow correct work of applications and
services via OLEDB on Windows. Here are some:

1. data_type field contains types names that are not recognized by MS apps.

Code around: rename types on the fly, e.g.

integer -> int

character varying -> varchar

character -> char

timestamp without time zone -> datetime

bytea -> image

2. character_maximum_length field

Code around: change value for text abd bytea types

[text] 1073741823

[bytea] 2147483647

3. character_octet_length should always be double of
character_maximum_length (due to Unicode character size on Windows which is
2).

4. datetime_precision field is not always correct

Code around: change value of the fly, e.g. if value is not null then

[numeric] keep the value (ok)

[bigint] set value to 19

all other set to 10

5. numeric_precision_radix field should always be equal to 10

6. datetime_precision field, minor changes

Code around: change value on the fly, e.g.

[date] set value to zero

[datetime] set value to 3