Additional ODBC patch

Further to the previous ODBC patches I posted today, I found a couple of
problems with char array sizes having set a couple of constants to 0 for
unlimited query length and row length. This additional patch cleans those
problems up by defining a new constant (STD_STATEMENT_LEN) to 65536 and
using that in place of MAX_STATEMENT_LEN.

Another constant (MAX_MESSAGE_LEN) was defined as 2*BLCKSZ, but is now
65536. This is used to define the length of the message buffer in a number
of places and as I understand it (probably not that well!) therefore also
places a limit on the query length. Fixing this properly is beyond my
capabilities but 65536 should hopefully be large enough for most people.

Apologies for being over-enthusiastic and posting 3 patches in one day
rather than 1 better tested one!

Regards,

Dave.

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