Proposed patch for ODBC driver w/ C-a-n-c-e-l

Patch against 7,2 submitted for comment.

It's a little messy; I had some trouble trying to reconcile the code
style of libpq which I copied from, and odbc.

Suggestions on what parts look ugly, and or where to send this
(is there a separate ODBC place?) are welcome.

This seems to work just fine; Now, when our users submit a 2 hour
query with four million row sorts by accident, then cancel it 30 seconds
later, it doesn't bog down the server ...

regards,

-Brad