Deal with the case that the buffer for SQLGetData is too small,
and also with missing trailing zeros.

To avoid loss of precision

Some types (e.g. PostGIS geometry columns which are mapped to text)
may have huge sized. We now use type text for those large columns
and limit the buffer size (which is no problem since
GetData by parts was implemented)

* Non-supported column types (including bit-strings) are now omitted
* bit(1) is interpreted as boolean
* Conversion to bytea (longvarbinary) is handled properly

The buf_used variable has been added to keep track of the buffer size
in case of binary data (non null-terminated) so it will be available
for future binary conversions.