Improve pgxalib.dll(MSDTC support).
1. Add an option which lets xa_open() try to connect to the database
   immediately. This would cause an error at enlistment in dtc when
   the authentication method depends not only on the db user but also
   on the OS user, which means the automatic transaction recovery is
   unavailable.
2. Log the error message in case of SQLDriverConnect() error.
3. Improve the logging.
Add an option to explicitly force/avoid the use of libpq library.
SSL connection with DTC.